Fix "Combine bit test + conditional or into simple math"

This fixes the optimization introduced in r179748 and reverted in r179750.

While the optimization was sound, it did not properly respect differences in
bit-width.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180777 91177308-0d34-0410-b5e6-96231b3b80d8