RFR: JDK11-only: Use WB-based acmp barrier

[Roman Kennke]

Let's keep the 'if (UseShenandoahGC && ShenandoahAcmpBarrier)' part:

http://cr.openjdk.java.net/~rkennke/acmp-shjdk11/webrev.01/

Roman


> This makes acmp barriers in sh/jdk11 to be like in jdk12, using simple
> WBs instead of complicated RBs.
> 
> Testing: tier3_gc_shenandoah
> 
> http://cr.openjdk.java.net/~rkennke/acmp-shjdk11/webrev.00/
> 
> Roland: can you please check if that covers all acmp-related stuff?
> 
> Otherwise, ok?
> 
> Roman
>