compiler: fix cse with different conditionals

[mloubout]

Fix cse to make sure expression from different conditionals get properly handled.

avoids things like

if (a){
  f1 = expr
 }
if (b){
  f1 = expr
 }

to be converted to

if (a){
  r0 = expr
  f1 = r0
 }
if (b){
  f1 = r0
 }

CSE now counts expression based on the equation's conditional. We could also just lift all rxx out of conditionals but that would be computing those cse temps way more than needed which I don't think it great

[codecov]

Codecov Report

Attention: Patch coverage is 96.29630% with 2 lines in your changes missing coverage. Please review.

Project coverage is 86.75%. Comparing base (6e79f1c) to head (a9a4d14).

Files	Patch %	Lines
devito/passes/clusters/cse.py	88.23%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2410   +/-   ##
=======================================
  Coverage   86.74%   86.75%           
=======================================
  Files         235      235           
  Lines       44563    44610   +47     
  Branches     8250     8257    +7     
=======================================
+ Hits        38657    38702   +45     
- Misses       5187     5189    +2     
  Partials      719      719           

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