RFR: fix aarch64 membar elision with shenandoah

[Roland Westrelin]

> Oh wait, I just got this from a test:
>
> #  Internal Error
> (/home/rkennke/src/shenandoah-jdk/src/hotspot/cpu/aarch64/aarch64.ad:2873),
> pid=27969, tid=28271
> #  guarantee(mbar != NULL) failed: CAS not embedded in normal graph!

With this test we're running with:

-XX:+ShenandoahSATBBarrier -XX:+ShenandoahWriteBarrier -XX:+ShenandoahReadBarrier -XX:+ShenandoahStoreValEnqueueBarrier

so we have a write barrier, an enqueue barrier and a satb barrier above
the CAS. Why does it make sense to run with options that are not usually
enabled together?

Roland.