rewrite hybrid SLP detection

rewrite hybrid SLP detection

This rewrites hybrid SLP detection to be simpler and cope with
group size changes in the SLP graph.  In particular detection
works starting from non-SLP stmts following use->def chains
rather than walking the SLP graph and following def->use chains.

It's all temporary of course since non-SLP and thus hybrid SLP
will go away.

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

	* tree-vect-slp.c (struct vdhs_data): New.
	(vect_detect_hybrid_slp): New walker.
	(vect_detect_hybrid_slp): Rewrite.