[ValueTracking, InstCombine] extend isKnownToBeAPowerOfTwo() to handle vector splat constants

We could try harder to handle non-splat vector constants too, 
but that seems much rarer to me.

Note that the div test isn't resolved because there's a check
for isIntegerTy() guarding that transform.

Differential Revision: http://reviews.llvm.org/D20497

llvm-svn: 270369