Use vectype from SLP node for vect_get_{load,store}_cost if possible

Use vectype from SLP node for vect_get_{load,store}_cost if possible

The vect_get_{load,store}_cost API is used from both vectorizable_*
where we've done SLP analysis and from alignment peeling analysis
with is done before this and thus only stmt_vec_infos are available.
The following patch makes sure we pick the vector type relevant
for costing from the SLP node when available.

	* tree-vect-stmts.cc (vect_get_store_cost): Compute vectype based
	on whether we got SLP node or stmt_vec_info and use the full
	record_stmt_cost API.
	(vect_get_load_cost): Likewise.