[SelectionDAG] rename/move isKnownToBeAPowerOfTwo() from TargetLowering (NFC)

There are at least 2 places (DAGCombiner, X86ISelLowering) where this could be used instead
of ad-hoc and watered down code that is trying to match a power-of-2 pattern.

Differential Revision: http://reviews.llvm.org/D20439

llvm-svn: 270073