vect: Move vect_model_load_cost next to the transform in vectorizable_load

vect: Move vect_model_load_cost next to the transform in vectorizable_load

This patch is an initial patch to move costing next to the
transform, it still adopts vect_model_load_cost for costing
but moves and duplicates it down according to the handlings
of different vect_memory_access_types, 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_load): Move and duplicate the call
	to vect_model_load_cost down to some different transform paths
	according to the handlings of different vect_memory_access_types.