re PR tree-optimization/42906 (Empty loop not removed)

	PR tree-optimization/42906

	* tree-ssa-dce.c (mark_control_dependent_edges_necessary): Add IGNORE_SELF
	argument; set visited_control_parents for fully processed BBs.
	(find_obviously_necessary_stmts): Update call of
	mark_control_dependent_edges_necessary.
	(propagate_necessity): Likewise; handle PHI edges more curefully.

	* gcc.dg/tree-ssa/dce-1.c: New testcase.

From-SVN: r158004