Avoid SSA copies from vect_add_slp_permutation

In the past we needed a stmt_vec_info for all vector defs.  Not
anymore.

	* tree-vect-slp.cc (vect_add_slp_permutation): We no longer
	need a copy when the extraction is readily available.
	(vect_schedule_slp_node): Handle default defs or constants
	in vector defs.