RFR (S) 8224881: Shenandoah: trashing "Collection Set, Pinned" region during Degenerated GC

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8224881

The failure is extremely hard to reproduce: I spent many hours trying, to no avail. It requires a
few unlikely things to happen at once. I managed to reproduce it a few times locally with the large
test, and the fix seems to help it. So, I would like to push the fix and see if it reproduces in the
upcoming months.

Fix:
  http://cr.openjdk.java.net/~shade/8224881/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey