[X86][AVX] Generalized matching for target shuffle combines

This patch is a first step towards a more extendible method of matching combined target shuffle masks.

Initially this just pulls out the existing basic mask matches and adds support for some 256/512 bit equivalents. Future patterns will require a number of features to be added but I wanted to keep this patch simple.

I hope we can avoid duplication between shuffle lowering and combining and share more complex pattern match functions in future commits.

Differential Revision: http://reviews.llvm.org/D19198

llvm-svn: 270230