InstSimplify: Handle some simple tautological comparisons

This handles cases where we are comparing a masked value against itself.
The analysis could be further improved by making it recursive but such
expense is not currently justified.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222716 91177308-0d34-0410-b5e6-96231b3b80d8