In Shenandoah, we currently start cleanup phase when we dive into concurrent evacuation (and only then), and after the cycle (unconditionally). However, we only create trashed regions in final-mark (immediate garbage and/or garbage from previous cycle), and might skip evacuation altogether, and may not actually create trash at end of cycle if we merge update-refs onto subsequent concurrent-mark. Let's start cleanups where we actually need them: once unconditionally before conc-evac and once after final-update-refs.