vect: Adjust vectorizable_load costing on VMAT_ELEMENTWISE and VMAT_STRIDED_SLP

vect: Adjust vectorizable_load costing on VMAT_ELEMENTWISE and VMAT_STRIDED_SLP

This patch adjusts the cost handling on VMAT_ELEMENTWISE
and VMAT_STRIDED_SLP in function vectorizable_load.  We
don't call function vect_model_load_cost for them any more.

As PR82255 shows, we don't always need a vector construction
there, moving costing next to the transform can make us only
cost for vector construction when it's actually needed.
Besides, it can count the number of loads consistently for
some cases.

	 PR tree-optimization/82255

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_load): Adjust the cost handling
	on VMAT_ELEMENTWISE and VMAT_STRIDED_SLP without calling
	vect_model_load_cost.
	(vect_model_load_cost): Assert it won't get VMAT_ELEMENTWISE and
	VMAT_STRIDED_SLP any more, and remove their related handlings.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/ppc/costmodel-pr82255.c: New test.

2023-06-13  Bill Schmidt  <wschmidt@linux.ibm.com>
	    Kewen Lin  <linkw@linux.ibm.com>