RFR: Shenandoah: Isolate changes around BarrierSetC2::enqueue_useful_gc_barrier()

[Roman Kennke]

On Wed, 2020-07-08 at 12:01 +0200, Roland Westrelin wrote:
> (I commented on the wrong RFR before. Back to the right one)
> 
> > Alright, so that is sufficient then?
> > 
> > http://cr.openjdk.java.net/~rkennke/shjdk11-c2-enqueue-barrier/webrev.01/
> 
> Yes, I think it's good. If this is somehow wrong, testing should
> catch it.

Alright, I've run hotspot_gc_shenandoah, and I'll also run CTW tests
before pushing it.

Thanks,
Roman