RFR: AArch64: Don't generate CAS barrier when -XX:-ShenandoahCASBarrier

[Roman Kennke]

Some tests are failing in aarch64 because we generate CAS barrier even
when passing -XX:-ShenandoahCASBarrier in ShBSA:

http://cr.openjdk.java.net/~rkennke/aarch64-fixcas/webrev.00/

Testing: manually checking failing tests

Ok?
Roman