RFR (S) 8231667: Shenandoah: Full GC should take empty regions into slices for compaction

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8231667

Fix:
  https://cr.openjdk.java.net/~shade/8231667/wevrev.01/

There is a problem with current Full GC that makes some tests fail with OOME unnecessarily. See
details in the bug report.

Testing: {x86_64, x86_32} hotspot_gc_shenandoah, affected tests

-- 
Thanks,
-Aleksey