[14] RFR 8242602: Shenandoah: allow earlier recycle of trashed regions during concurrent root processing

[Zhengyu Gu]

Hi,

I would like to backport this patch to 14u. Early recycling immediate 
trash regions, provides additional head room for GC to avoid allocation 
failures, especially, when concurrent class unloading can take quite 
some time.

The original patch does not apply cleanly, but the conflicts are minor, 
due to line shifts in ShenandoahHeap.cpp.

Original Bug: https://bugs.openjdk.java.net/browse/JDK-8242602
Original Webrev: http://cr.openjdk.java.net/~zgu/JDK-8242602/webrev.01/

14u Webrev: http://cr.openjdk.java.net/~zgu/JDK-8242602-14u/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu