vect: Consider vec_perm costing for VMAT_CONTIGUOUS_REVERSE

vect: Consider vec_perm costing for VMAT_CONTIGUOUS_REVERSE

For VMAT_CONTIGUOUS_REVERSE, the transform code in function
vectorizable_store generates a VEC_PERM_EXPR stmt before
storing, but it's never considered in costing.

This patch is to make it consider vec_perm in costing, it
adjusts the order of transform code a bit to make it easy
to early return for costing_p.

gcc/ChangeLog:

	* tree-vect-stmts.cc (vectorizable_store): Consider generated
	VEC_PERM_EXPR stmt for VMAT_CONTIGUOUS_REVERSE in costing as
	vec_perm.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/costmodel/ppc/costmodel-vect-store-2.c: New test.