[X86][AVX] Add combineConcatVectors support to improve subvector handling

Attempt to combine CONCAT_VECTORS nodes, which we only really have pre-legalization.

This encourages a lot of X86ISD::SUBV_BROADCAST generation, so I've added SimplifyDemandedVectorEltsForTargetNode handling for this at the same time.

The X86ISD::VTRUNC regression in shuffle-vs-trunc-256-widen.ll will be handled in a future commit.

llvm-svn: 356064