vect: Move vect_model_store_cost next to the transform in vectorizable_store

vect: Move vect_model_store_cost next to the transform in vectorizable_store

This patch is an initial patch to move costing next to the
transform, it still adopts vect_model_store_cost for costing
but moves and duplicates it down according to the handlings
of different vect_memory_access_types or some special
handling need, hope it can make the subsequent patches easy
to review.  This patch should not have any functional
changes.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_store): Move and duplicate the call
	to vect_model_store_cost down to some different transform paths
	according to the handlings of different vect_memory_access_types
	or some special handling need.