DAGCombiner: Combine SDIV with non-splat vector pow2 divisor

Summary:
Extend existing SDIV combine for pow2 constant divider to handle
non-splat vectors of pow2 constants.

Reviewers: RKSimon, craig.topper, spatel, hfinkel, efriedma

Reviewed By: RKSimon

Subscribers: magabari, llvm-commits

Differential Revision: https://reviews.llvm.org/D42479