[DAGCombine] narrowInsertExtractVectorBinOp - add CONCAT_VECTORS support

We already split extract_subvector(binop(insert_subvector(v,x),insert_subvector(w,y))) -> binop(x,y).

This patch adds support for extract_subvector(binop(concat_vectors(),concat_vectors())) cases as well.

In particular this means we don't have to wait for X86 lowering to convert concat_vectors to insert_subvector chains, which helps avoid some cases where demandedelts/combine calls occur too late to split large vector ops.

The fast-isel-store.ll load folding regression is annoying but I don't think is that critical.

Differential Revision: https://reviews.llvm.org/D63653

llvm-svn: 365785