[X86][AVX] Pull out some INSERT_SUBVECTOR combines into a combineConcatVectorOps helper. NFCI

A lot of the INSERT_SUBVECTOR combines can be more generally handled as if they have come from a CONCAT_VECTORS node.

I've been investigating adding a CONCAT_VECTORS combine to X86, but this is a much easier first step that avoids the issue of handling a number of pre-legalization issues that I've encountered.

Differential Revision: https://reviews.llvm.org/D58583

llvm-svn: 355015