RFR: Disable barrier optimization on constant when +ShenandoahBarriersForConst

[Roman Kennke]

There is an optimization in
CmpPNode::shenandoah_optimize_java_mirror_cmp() that eliminates barriers
when comparing two java mirror objects. We need those barriers when
running with +ShenandoahBarriersForConst. This patch disables that
optimization:

http://cr.openjdk.java.net/~rkennke/fixbarriersforconst/webrev.00/
<http://cr.openjdk.java.net/%7Erkennke/fixbarriersforconst/webrev.00/>

Ok?

Roman