[InstCombine] rename variables in foldICmpDivConstant(); NFC

Removing the redundant 'CmpRHSV' local variable exposes a bug in the caller
foldICmpShrConstant() - it was sending in the div constant instead of the
cmp constant. But I have not been able to expose this in a regression test
yet - the affected folds all appear to be handled before we ever reach this
code. I'll keep trying to find a case as I make changes to allow vector folds
in both functions.

llvm-svn: 279828