RFR: JDK-8217016: Shenandoah: Streamline generation of CAS barriers

[Roman Kennke]

> http://cr.openjdk.java.net/~rkennke/JDK-8217016/webrev.00/
> 
> That looks good to me. But don't you need a similar change for aarch64?

Right. This update implements the same thing in aarch64:

http://cr.openjdk.java.net/~rkennke/JDK-8217016/webrev.01/

It causes no regressions in hotspot_gc_shenandoah (we have a few
failures, but none of them are related to the patch and are pre-existing).

Good?

Thanks,
Roman