tree-optimization/100923 - fix alias-ref construction wrt availability

tree-optimization/100923 - fix alias-ref construction wrt availability

This PR shows that building an ao_ref from value-numbers is prone to
expose bogus contextual alias info to the oracle.  The following makes
sure to construct ao_refs from SSA names available at the program point
only.

On the way it modifies the awkward valueize_refs[_1] API.

2021-06-08  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/100923
	* tree-ssa-sccvn.c (valueize_refs_1): Take a pointer to
	the operand vector to be valueized.
	(valueize_refs): Likewise.
	(valueize_shared_reference_ops_from_ref): Adjust.
	(valueize_shared_reference_ops_from_call): Likewise.
	(vn_reference_lookup_3): Likewise.
	(vn_reference_lookup_pieces): Likewise.  Re-valueize
	with honoring availability when we are about to create
	the ao_ref and valueized before.
	(vn_reference_lookup): Likewise.
	(vn_reference_insert_pieces): Adjust.

	* gcc.dg/torture/pr100923.c: New testcase.

(cherry picked from commit 7a56d3d3e99cc77ad8a6a674870c814da6225675)