tree-optimization/101088 - fix SM invalidation issue

tree-optimization/101088 - fix SM invalidation issue

When we face a sm_ord vs sm_unord for the same ref during
store sequence merging we assert that the ref is already marked
unsupported.  But it can be that it will only be marked so
during the ongoing merging so instead of asserting mark it here.

Also apply some optimization to not waste resources to search
for already unsupported refs.

2021-06-16  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101088
	* tree-ssa-loop-im.c (sm_seq_valid_bb): Only look for
	supported refs on edges.  Do not assert same ref but
	different kind stores are unsuported but mark them so.
	(hoist_memory_references): Only look for supported refs
	on exits.

	* gcc.dg/torture/pr101088.c: New testcase.