RFR: JDK-8231499: Shenandoah: Fix and simplify clone barrier

[Roman Kennke]

Hi Alexsey,

>> Webrev:
>> http://cr.openjdk.java.net/~rkennke/JDK-8231499/webrev.01/
> 
> *) Synopsis should be updated? I think something like:
>  "Shenandoah: Fix and simplify clone barrier"

Right. Thanks!

> *) Is there no way to reuse ShenandoahBarrierC2Support::test_heap_stable somehow?

I don't think so. test_heap_stable() uses PhaseIdealLoop,
clone-expansion uses PhaseMacroExpand. Abstracting this would make
things even more complicated.

> If not, this is my round of cleanups on top of that patch:
>   https://cr.openjdk.java.net/~shade/shenandoah/8231499-shade-cleanup.patch

Thanks!

Folded patch:
http://cr.openjdk.java.net/~rkennke/JDK-8231499/webrev.03/

Would like to have Roland look at it too.

Thanks,
Roman