[DAGCombiner] Match non-uniform constant vectors using predicates.

Most combines currently recognise scalar and splat-vector constants, but not non-uniform vector constants.

This patch introduces a matching mechanism that uses predicates to check against BUILD_VECTOR of ConstantSDNode, as well as scalar ConstantSDNode cases.

I've changed a couple of predicates to demonstrate - the combine-shl changes add currently unsupported cases, while the MatchRotate replaces an existing mechanism.

Differential Revision: https://reviews.llvm.org/D35492

llvm-svn: 308598