[x86] Sink a generic combine of VZEXT nodes from the lowering to VZEXT

nodes to the DAG combining of them.

This will allow the combine to fire on both old vector shuffle lowering
and the new vector shuffle lowering and generally seems like a cleaner
design. I've trimmed down the code a bit and tried to make it and the
surrounding combine fairly clean while moving it around.

llvm-svn: 219042