[SelectionDAG] fold insert subvector of undef into undef

DAGCombiner simplifies this more liberally as:
  // If inserting an UNDEF, just return the original vector.
  if (N1.isUndef())
    return N0;

So there's no way to make this visible in output AFAIK, but
doing this at node creation time should be slightly more efficient.

llvm-svn: 361287