tree-optimization/115388 - wrong DSE in irreductible regions

tree-optimization/115388 - wrong DSE in irreductible regions

The following fixes a latent bug in DSE with regarding to variant
array accesses where the code avoiding bogus DSE in loops fails to
handle irreducible regions.  For those we need to make sure backedges
are marked and discover a header for the irreducible region to check
invariantness.

	PR tree-optimization/115388
	* tree-ssa-dse.cc (dse_classify_store): Handle irreducible
	regions.
	(pass_dse::execute): Make sure to mark backedges.

	* gcc.dg/torture/pr115388.c: New testcase.