vect: Adjust vectorizable_store costing on VMAT_LOAD_STORE_LANES

This patch adjusts the cost handling on VMAT_LOAD_STORE_LANES
in function vectorizable_store.  We don't call function
vect_model_store_cost for it any more.  It's the case of
interleaving stores, so it skips all stmts excepting for
first_stmt_info, consider the whole group when costing
first_stmt_info.  This patch shouldn't have any functional
changes.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vect_model_store_cost): Assert it will never
	get VMAT_LOAD_STORE_LANES.
	(vectorizable_store): Adjust the cost handling on VMAT_LOAD_STORE_LANES
	without calling vect_model_store_cost.  Factor out new lambda function
	update_prologue_cost.