[CGP] refactor optimizeCmpExpression (NFCI)

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352988 91177308-0d34-0410-b5e6-96231b3b80d8