RFR: Implement ShenandoahHeap::requires_barriers()

[Ron Pressler]

On Fri, 12 Feb 2021 20:39:26 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> This add proper implementation of requires_barriers() for ShenandoahHeap, and adds some comments to CollectedHeap about it.
> 
> Testing (with a few other fixes in the pipeline):
>  - [x] java/lang/Continuation (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)
>  - [x] java/lang/Thread/virtual  (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)

Marked as reviewed by rpressler (Lead).

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PR: https://git.openjdk.java.net/loom/pull/29