[InstCombine] clean up foldICmpDivConstant; NFCI

1. Fix comments to match variable names
2. Remove redundant CmpRHS variable
3. Add FIXME to replace some checks with asserts


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280112 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed