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

[Roman Kennke]

>> 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/
> 
> Upon review, I realized this got into jdk/jdk after fork to jdk13:
>   http://hg.openjdk.java.net/jdk/jdk/rev/2c47220ce9bb
> 
> I would like to have it in jdk13 too. Okay to push to jdk/jdk13 too?

Good to me.

Roman