Optimize submul part of udivrem

[chfast]

Inline subc() in submul() and use borrow flag idiom
which is recognizable by GCC.

[chfast]

div<uint256, udivrem>/64                                      -0.0036         -0.0036            23            23            23            23                                                                                     
div<uint256, udivrem>/128                                     +0.0026         +0.0026            29            29            29            29                                                                                     
div<uint256, udivrem>/192                                     -0.0414         -0.0414            34            33            34            33                                                                                     
div<uint256, udivrem>/256                                     -0.0468         -0.0468            30            28            30            28                                                                                     
div<uint512, udivrem>/64                                      -0.0025         -0.0025            37            37            37            37                                                                                     
div<uint512, udivrem>/128                                     -0.0144         -0.0145            49            48            49            48                                                                                     
div<uint512, udivrem>/192                                     -0.0440         -0.0440            72            68            72            68                                                                                     
div<uint512, udivrem>/256                                     -0.0480         -0.0480            69            66            69            66                                                                                     
mod<addmod>/64                                                -0.0115         -0.0115            30            29            30            29                                                                                     
mod<addmod>/128                                               -0.0041         -0.0041            33            33            33            33                                                                                     
mod<addmod>/192                                               -0.0133         -0.0133            41            40            41            40                                                                                     
mod<addmod>/256                                               -0.0111         -0.0111            35            35            35            35                                                                                     
mod<addmod_public>/64                                         -0.0054         -0.0054            33            33            33            33                                                                                     
mod<addmod_public>/128                                        -0.0044         -0.0044            37            36            37            36                                                                                     
mod<addmod_public>/192                                        -0.0403         -0.0403            44            42            44            42                                                                                     
mod<addmod_public>/256                                        -0.0279         -0.0279            37            36            37            36                                                                                     
mod<addmod_simple>/64                                         +0.0046         +0.0046            32            32            32            32                                                                                     
mod<addmod_simple>/128                                        +0.0131         +0.0131            34            35            34            35                                                                                     
mod<addmod_simple>/192                                        -0.0250         -0.0250            41            40            41            40                                                                                     
mod<addmod_simple>/256                                        -0.0008         -0.0008            35            35            35            35                                                                                     
mod<addmod_prenormalize>/64                                   -0.0137         -0.0137            66            65            66            65                                                                                     
mod<addmod_prenormalize>/128                                  +0.0170         +0.0170            69            70            69            70                                                                                     
mod<addmod_prenormalize>/192                                  -0.0308         -0.0308            81            79            81            79                                                                                     
mod<addmod_prenormalize>/256                                  -0.0205         -0.0205            62            61            62            61                                                                                     
mod<addmod_daosvik>/64                                        -0.0034         -0.0034            33            32            33            32                                                                                     
mod<addmod_daosvik>/128                                       -0.0102         -0.0102            36            36            36            36                                                                                     
mod<addmod_daosvik>/192                                       -0.0448         -0.0448            43            41            43            41                                                                                     
mod<addmod_daosvik>/256                                       -0.0248         -0.0248            36            35            36            35                                                                                     
mod<mulmod>/64                                                +0.0123         +0.0123            49            49            49            49                                                                                     
mod<mulmod>/128                                               +0.0191         +0.0191            59            60            59            60                                                                                     
mod<mulmod>/192                                               -0.0710         -0.0710            85            79            85            79                                                                                     
mod<mulmod>/256                                               -0.0648         -0.0648            82            77            82            77                                                                                     
ecmod<addmod_public>                                          +0.0164         +0.0163            19            19            19            19                                                                                     
ecmod<addmod_simple>                                          -0.0050         -0.0050            26            26            26            26                                                                                     
ecmod<addmod_prenormalize>                                    -0.0016         -0.0016            10            10            10            10                                                                                     
ecmod<addmod_daosvik>                                         +0.0006         +0.0006            18            18            18            18                                                                                     
ecmod<mulmod>                                                 -0.0508         -0.0508            79            75            79            75                                                                                     

[codecov-commenter]

Codecov Report

Merging #263 (791d1c5) into master (b05e8a1) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #263   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         1911      1911           
=========================================
  Hits          1911      1911           

Flag	Coverage Δ
32bit	100.00% <100.00%> (ø)
gcc	100.00% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
include/intx/intx.hpp	100.00% <100.00%> (ø)