tree-optimization/97626 - handle SCCs properly in SLP stmt analysis

tree-optimization/97626 - handle SCCs properly in SLP stmt analysis

This makes sure to roll-back the whole SCC when we fail stmt
analysis, otherwise the optimistic visited treatment breaks down
with different entries.  Rollback is easy when tracking additions
to visited in a vector which also makes the whole thing cheaper
than the two hash-sets used before.

2020-10-30  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/97626
	* tree-vect-slp.c (vect_slp_analyze_node_operations):
	Exchange the lvisited hash-set for a vector, roll back
	recursive adds to visited when analysis failed.
	(vect_slp_analyze_operations): Likewise.

	* gcc.dg/vect/bb-slp-pr97626.c: New testcase.