Prepare removal of SLP_INSTANCE_GROUP_SIZE

Prepare removal of SLP_INSTANCE_GROUP_SIZE

This removes trivial instances of SLP_INSTANCE_GROUP_SIZE and refrains
from using a "SLP instance" which nowadays is just one of the possibly
many entries into the SLP graph.

2020-05-06  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (vect_transform_slp_perm_load): Adjust.
	* tree-vect-data-refs.c (vect_slp_analyze_node_dependences):
	Remove slp_instance parameter, just iterate over all scalar stmts.
	(vect_slp_analyze_instance_dependence): Adjust and likewise.
	* tree-vect-slp.c (vect_bb_slp_scalar_cost): Remove unused BB
	parameter.
	(vect_schedule_slp): Just iterate over all scalar stmts.
	(vect_supported_load_permutation_p): Adjust.
	(vect_transform_slp_perm_load): Remove slp_instance parameter,
	instead use the number of lanes in the node as group size.
	* tree-vect-stmts.c (vect_model_load_cost): Get vectorization
	factor instead of slp_instance as parameter.
	(vectorizable_load): Adjust.