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

This causes crashes during the build of compiler-rt during selfhost. Add a
testcase for coverage.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173279 91177308-0d34-0410-b5e6-96231b3b80d8