[DAGCombiner] Extend `combineFMulOrFDivWithIntPow2` to work for non-splat float vecs

Do so by extending `matchUnaryPredicate` to also work for
`ConstantFPSDNode` types then encapsulate the constant checks in a
lambda and pass it to `matchUnaryPredicate`.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D154868