tree-optimization/100810 - avoid undefs in IVOPT rewrites

tree-optimization/100810 - avoid undefs in IVOPT rewrites

The following attempts to avoid IVOPTs rewriting uses using
IV candidates that involve undefined behavior by using uninitialized
SSA names.  First we restrict the set of candidates we produce
for such IVs to the original ones and mark them as not important.
Second we try to only allow expressing uses with such IV if they
originally use them.  That is to avoid rewriting all such uses
in terms of other IVs.  Since cand->iv and use->iv seem to never
exactly match up we resort to comparing the IV bases.

The approach ends up similar to the one posted by Roger at
https://gcc.gnu.org/pipermail/gcc-patches/2021-August/578441.html
but it marks IV candidates rather than use groups and the cases
we allow in determine_group_iv_cost_generic are slightly different.

2022-01-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100810
	* tree-ssa-loop-ivopts.cc (struct iv_cand): Add involves_undefs flag.
	(find_ssa_undef): New function.
	(add_candidate_1): Avoid adding derived candidates with
	undefined SSA names and mark the original ones.
	(determine_group_iv_cost_generic): Reject rewriting
	uses with a different IV when that involves undefined SSA names.

	* gcc.dg/torture/pr100810.c: New testcase.
	* gcc.dg/torture/pr105337.c: Likewise.