vect: Get rid of vect_model_store_cost

This patch is to eventually get rid of vect_model_store_cost,
it adjusts the costing for the remaining memory access types
VMAT_CONTIGUOUS{, _DOWN, _REVERSE} by moving costing close
to the transform code.  Note that in vect_model_store_cost,
there is one special handling for vectorizing a store into
the function result, since it's extra penalty and the
transform part doesn't have it, this patch keep it alone.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vect_model_store_cost): Remove.
	(vectorizable_store): Adjust the costing for the remaining memory
	access types VMAT_CONTIGUOUS{, _DOWN, _REVERSE}.