RFR (S) 8222786: Shenandoah get_barrier_strength should accept all shapes of (Weak)CAS reference barriers

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8222786

Fix:
  http://cr.openjdk.java.net/~shade/8222786/webrev.01/

See bug for the example failures. I am puzzled why those were not covered before.

Testing: hotspot_gc_shenandoah, jcstress

-- 
Thanks,
-Aleksey