Rewrite the int128 clamp to avoid a branch

[charles-cooper]

and also avoid direct manipulation of the stack

I guess this isn't as important now that the standard is to use int256.

How I did it

Bit twiddling

How to verify it

Tests still pass

Description for the changelog

Increase efficiency of int128 clamps

Cute Animal Picture

[codecov-commenter]

Codecov Report

Merging #2411 (febc8c6) into master (b52715e) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2411      +/-   ##
==========================================
+ Coverage   85.47%   85.51%   +0.03%     
==========================================
  Files          91       91              
  Lines        9034     9034              
  Branches     2151     2151              
==========================================
+ Hits         7722     7725       +3     
+ Misses        806      804       -2     
+ Partials      506      505       -1     

Impacted Files	Coverage Δ
vyper/old_codegen/arg_clamps.py	100.00% <ø> (ø)
vyper/builtin_functions/functions.py	88.68% <0.00%> (+0.36%)	⬆️

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[iamdefinitelyahuman]

Looks good. And the clamping is well tested so if there was an issue we'd see failing tests.