tree-optimization/105726 - adjust array bound heuristic

tree-optimization/105726 - adjust array bound heuristic

There's heuristic to detect ptr[1].a[...] out of bound accesses
reasoning that if ptr points to an array of aggregates a trailing
incomplete array has to have size zero.  The following more
thoroughly constrains the cases this applies to avoid false
positive diagnostics.

2022-05-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105726
	* gimple-ssa-warn-restrict.c (builtin_memref::set_base_and_offset):
	Constrain array-of-flexarray case more.

	* g++.dg/warn/Warray-bounds-27.C: New testcase.

(cherry picked from commit e7c482b08076bb299742883c4ffd65b31e33200c)