isKnownToBeAPowerOfTwo: Strengthen isKnownToBeAPowerOfTwo's analysis on add instructions

Call into ComputeMaskedBits to figure out which bits are set on both add
operands and determine if the value is a power-of-two-or-zero or not.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187445 91177308-0d34-0410-b5e6-96231b3b80d8