middle-end: Store and use the SLP instance kind when aborting load/store lanes

middle-end: Store and use the SLP instance kind when aborting load/store lanes

This patch stores the SLP instance kind in the SLP instance so that we can use
it later when detecting load/store lanes support.

This also changes the load/store lane support check to only check if the SLP
kind is a store.  This means that in order for the load/lanes to work all
instances must be of kind store.

gcc/ChangeLog:

	* tree-vect-loop.c (vect_analyze_loop_2): Check kind.
	* tree-vect-slp.c (vect_build_slp_instance): New.
	(enum slp_instance_kind): Move to...
	* tree-vectorizer.h (enum slp_instance_kind): .. Here
	(SLP_INSTANCE_KIND): New.