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

[Roman Kennke]

Looks good. Thanks!

Roman

> 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
>