RFR(T) 8230350: Shenandoah: Assertion failed when GC is cancelled by a worker thread

[Zhengyu Gu]

This assertion failure rarely happens, because 
ShenandoahSuspendibleWorkers is off by default, so this path is rarely 
executed in jdk/jdk.

Now, ShenandoahSuspendibleWorkers is on when concurrent class unloading 
is enabled in shenandoah/jdk, but I only saw this once, so it is still rare.

Bug: https://bugs.openjdk.java.net/browse/JDK-8230350
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8230350/webrev.00/


Test:
   hotspot_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu