RFR: relax pattern matching for CAS on aarch64

[Roland Westrelin]

> How about this?
>   http://cr.openjdk.java.net/~shade/shenandoah/cas-pattern-matching/webrev.01/

Ok. But

2005     if (st && (!mm || (UseShenandoahGC && !mm2))) {
2006       return NULL;
2007     }

should be:

if (st && (!mm || (UseShenandoahGC && mm2))) {
  return NULL;
}

Roland.