ValueTracking: Teach isKnownToBeAPowerOfTwo about (ADD X, (XOR X, Y)) where X is a power of two

This allows us to simplify urem instructions involving the add+xor to
turn into simpler math.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185272 91177308-0d34-0410-b5e6-96231b3b80d8