tree-optimization/101278 - handle self-use in DSE analysis

tree-optimization/101278 - handle self-use in DSE analysis

DSE store classification short-cuts the to-be classified stmt itself
from chaining but fails to first check whether the store uses itself
which can be the case when it is a call with the LHS also passed by
value as argument.

2021-07-01  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/101278
	* tree-ssa-dse.c (dse_classify_store): First check for
	uses, then ignore stmt for chaining purposes.

	* gcc.dg/torture/pr101278.c: New testcase.