tree-optimization/106293 - missed DSE with virtual LC PHI

tree-optimization/106293 - missed DSE with virtual LC PHI

Degenerate virtual PHIs can break DSEs fragile heuristic as to what
defs it can handle for further processing.  The following enhances
it to look through degenerate PHIs by means of a worklist, processing
the degenerate PHI defs uses to the defs array.  The rewrite of
virtuals into loop-closed SSA caused this to issue appear more often.
The patch itself is mostly re-indenting the new loop body.

	PR tree-optimization/106293
	* tree-ssa-dse.cc (dse_classify_store): Use a worklist to
	process degenerate PHI defs.

	* gcc.dg/tree-ssa/ssa-dse-46.c: New testcase.