[x86] Expand the target DAG combining for PSHUFD nodes to be able to
combine into half-shuffles through unpack instructions that expand the
half to a whole vector without messing with the dword lanes.

This fixes some redundant instructions in splat-like lowerings for
v16i8, which are now getting to be *really* nice.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212695 91177308-0d34-0410-b5e6-96231b3b80d8