RFR: JDK-8216973: Shenandoah: Kick up cleanup phases in the right places

[Roman Kennke]

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.

http://cr.openjdk.java.net/~rkennke/JDK-8216973/webrev.00/

Testing: hotspot_gc_shenandoah

Also, should this go into JDK12?

Thanks,
Roman

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