This fixes a common mistake in removing a store that looks redudnant but
is not because it changes the dynamic type of the memory and thus makes
a difference for following loads with TBAA.

2020-03-03  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93946
	* alias.h (refs_same_for_tbaa_p): Declare.
	* alias.c (refs_same_for_tbaa_p): New function.
	* tree-ssa-alias.c (ao_ref_alias_set): For a NULL ref return
	zero.
	* tree-ssa-scopedtables.h
	(avail_exprs_stack::lookup_avail_expr): Add output argument
	giving access to the hashtable entry.
	* tree-ssa-scopedtables.c (avail_exprs_stack::lookup_avail_expr):
	Likewise.
	* tree-ssa-dom.c: Include alias.h.
	(dom_opt_dom_walker::optimize_stmt): Validate TBAA state before
	removing redundant store.
	* tree-ssa-sccvn.h (vn_reference_s::base_set): New member.
	(ao_ref_init_from_vn_reference): Adjust prototype.
	(vn_reference_lookup_pieces): Likewise.
	(vn_reference_insert_pieces): Likewise.
	* tree-ssa-sccvn.c: Track base alias set in addition to alias
	set everywhere.
	(eliminate_dom_walker::eliminate_stmt): Also check base alias
	set when removing redundant stores.
	(visit_reference_op_store): Likewise.
	* dse.c (record_store): Adjust valdity check for redundant
	store removal.

	* gcc.dg/torture/pr93946-1.c: New testcase.
	* gcc.dg/torture/pr93946-2.c: Likewise.