InstCombine: fold (A >> C) == (B >> C) --> (A^B) < (1 << C) for constant Cs.

This is common in bitfield code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194925 91177308-0d34-0410-b5e6-96231b3b80d8