tree-optimization/106722 - fix CD-DCE edge marking

tree-optimization/106722 - fix CD-DCE edge marking

The following fixes a latent issue when we mark control edges but
end up with marking a block with no stmts necessary.  In this case
we fail to mark dependent control edges of that block.

	PR tree-optimization/106722
	* tree-ssa-dce.cc (mark_last_stmt_necessary): Return
	whether we marked a stmt.
	(mark_control_dependent_edges_necessary): When
	mark_last_stmt_necessary didn't mark any stmt make sure
	to mark its control dependent edges.
	(propagate_necessity): Likewise.

	* gcc.dg/torture/pr108737.c: New testcase.