tree-optimization/105562 - avoid uninit diagnostic with better FRE

tree-optimization/105562 - avoid uninit diagnostic with better FRE

We can avoid some uninit diagnostics by making FRE disambiguate
against CLOBBERs since any aliasing there would invoke undefined
behavior for a read we are looking up.

2022-05-12  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105562
	* tree-ssa-sccvn.cc (vn_reference_lookup_3): Disambiguate
	against all CLOBBER defs if there's not an obvious must-alias
	and we are not doing redundant store elimination.
	(vn_walk_cb_data::redundant_store_removal_p): New field.
	(vn_reference_lookup_pieces): Initialize it.
	(vn_reference_lookup): Add argument to specify if we are
	doing redundant store removal.
	(eliminate_dom_walker::eliminate_stmt): Specify we do.
	* tree-ssa-sccvn.h (vn_reference_lookup): Adjust.

	* g++.dg/warn/uninit-pr105562.C: New testcase.

(cherry picked from commit 94b8a37fa16f7638cc1965718f4ec71719506743)