[SDAG] Rather than using a narrow test against the one dummy node on the
stack, filter all handle nodes from the DAG combiner worklist.

This will also handle cases where other handle nodes might be
(erroneously) added to the worklist and then cause bugs and explosions
when deleted. For example, when running the legalizer within the DAG
combiner, there are times when other handle nodes are used and can end
up here.