tree-optimization/104162 - CSE of &MEM[ptr].a[i] and ptr + CST

tree-optimization/104162 - CSE of &MEM[ptr].a[i] and ptr + CST

This adds the capability to value-numbering of treating complex
address expressions where the offset becomes invariant as equal
to a POINTER_PLUS_EXPR.  This restores CSE that is now prevented
by early lowering of &MEM[ptr + CST] to a POINTER_PLUS_EXPR.

Unfortunately this regresses gcc.dg/asan/pr99673.c again, so
the testcase is adjusted accordingly.

2022-01-26  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104162
	* tree-ssa-sccvn.cc (vn_reference_lookup): Handle
	&MEM[_1 + 5].a[i] like a POINTER_PLUS_EXPR if the offset
	becomes invariant.
	(vn_reference_insert): Likewise.

	* gcc.dg/tree-ssa/ssa-fre-99.c: New testcase.
	* gcc.dg/asan/pr99673.c: Adjust.