vect: Simplify costing on vectorizable_scan_store

This patch is to simplify the costing on the case
vectorizable_scan_store without calling function
vect_model_store_cost any more.

I considered if moving the costing into function
vectorizable_scan_store is a good idea, for doing
that, we have to pass several variables down which
are only used for costing, and for now we just
want to keep the costing as the previous, haven't
tried to make this costing consistent with what the
transforming does, so I think we can leave it for now.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_store): Adjust costing on
	vectorizable_scan_store without calling vect_model_store_cost
	any more.