tree-optimization/95268 - fix commoning of clobbers

This fixes handling of clobbers when commoning stores.

2020-05-22  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/95268
	* tree-ssa-sink.c (sink_common_stores_to_bb): Handle clobbers
	properly.

	* g++.dg/torture/pr95268.C: New testcase.