Fix availability compute during VN DOM elimination

Fix availability compute during VN DOM elimination

This fixes an issue with redundant store elimination in FRE/PRE
which, when invoked by the DOM elimination walk, ends up using
possibly stale availability data from the RPO walk.  It also
fixes a missed optimization during valueization of addresses
by making sure to use get_addr_base_and_unit_offset_1 which can
valueize and adjusting that to also valueize ARRAY_REFs low-bound.

2020-05-08  Richard Biener  <rguenther@suse.de>

	* tree-ssa-sccvn.c (rpo_avail): Change type to
	eliminate_dom_walker *.
	(eliminate_with_rpo_vn): Adjust rpo_avail to make vn_valueize
	use the DOM walker availability.
	(vn_reference_fold_indirect): Use get_addr_base_and_unit_offset_1
	with vn_valueize as valueization callback.
	(vn_reference_maybe_forwprop_address): Likewise.
	* tree-dfa.c (get_addr_base_and_unit_offset_1): Also valueize
	array_ref_low_bound.

	* gnat.dg/opt83.adb: New testcase.