RFR: Add missing case for CAE to ShLRB::needs_barrier_impl()

[Roman Kennke]

We need a case for CompareAndExchangeP|N in
ShenandoahLoadReferenceBarrierNode::needs_barrier_impl(), otherwise we
crash in the ShouldNotReachHere() below (in debug builds). This happened
in CI.

Testing: hotspot_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/lrb-cae-2/webrev.00/

Good?

Roman