InstCombine: Clean up weird code that talks about a modulus that's long gone.

This does the right thing unless the multiplication overflows, but the old code
didn't handle that case either.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173276 91177308-0d34-0410-b5e6-96231b3b80d8