InstCombine: Turn urem to bitwise-and more often

Use isKnownToBeAPowerOfTwo in visitUrem so that we may more aggressively
fold away urem instructions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181661 91177308-0d34-0410-b5e6-96231b3b80d8