Make vect_model_store_cost take a vec_load_store_type

Make vect_model_store_cost take a vec_load_store_type

This patch makes vect_model_store_cost take a vec_load_store_type
instead of a vect_def_type.  It's a wash on its own, but it helps
with later patches.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vectorizer.h (vec_load_store_type): Moved from tree-vec-stmts.c
	(vect_model_store_cost): Take a vec_load_store_type instead of a
	vect_def_type.
	* tree-vect-stmts.c (vec_load_store_type): Move to tree-vectorizer.h.
	(vect_model_store_cost): Take a vec_load_store_type instead of a
	vect_def_type.
	(vectorizable_mask_load_store): Update accordingly.
	(vectorizable_store): Likewise.
	* tree-vect-slp.c (vect_analyze_slp_cost_1): Update accordingly.

From-SVN: r256211