[InstCombine] move constant check into foldBinOpIntoSelectOrPhi; NFCI

Also, rename 'foldOpWithConstantIntoOperand' because that's annoyingly 
vague. The constant check is redundant in some cases, but it allows 
removing duplication for most of the calls.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@326329 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed