[X86][XOP] Add support for lowering vector rotations

[X86][XOP] Add support for lowering vector rotations

This patch adds support for lowering to the XOP VPROT / VPROTI vector bit rotation instructions.

This has required changes to the DAGCombiner rotation pattern matching to support vector types - so far I've only changed it to support splat vectors, but generalising this further is feasible in the future.

Differential Revision: http://reviews.llvm.org/D13851

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251188 91177308-0d34-0410-b5e6-96231b3b80d8