[CGP] refactor optimizeCmpExpression (NFCI)

This is not truly NFC because we are bailing out without
a TLI now. That should not be a real concern though because
there should be a TLI in any real-world scenario. 

That seems better than passing around a pointer and then 
checking it for null-ness all over the place.

The motivation is to fix what appears to be an unintended
restriction on the uaddo transform - 
hasMultipleConditionRegisters() shouldn't be reason to limit 
the transform.

llvm-svn: 352988