tree-optimization/93354 FRE redundant store removal validity fix

This fixes tracking of the alias-set of partial defs for use by
redundant store removal.

2020-01-23  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/93381
	* tree-ssa-sccvn.c (vn_walk_cb_data::push_partial_def): Take
	alias-set of the def as argument and record the first one.
	(vn_walk_cb_data::first_set): New member.
	(vn_reference_lookup_3): Pass the alias-set of the current def
	to push_partial_def.  Fix alias-set used in the aggregate copy
	case.
	(vn_reference_lookup): Consistently set *last_vuse_ptr.
	* real.c (clear_significand_below): Fix out-of-bound access.

	* gcc.dg/torture/pr93354.c: New testcase.