The entire ShenandoahDegenGC is executed at safepoint, the early cleanup seems unnecessary because `op_cleanup_complete` is always executed at the end of ShenandoahDegenGC after update refs, except when the degen is cancelled(we don't have to recycle the trashed regions in this case).

Meanwhile, it looks like ShenandoahDegenGC doesn't have phase timing for early cleanup, I don't how much this could save, I'll add the phase timing for early cleanup to collect some metrics.