RFR: Use BarrierSetC2::ideal_node() API for CmpPNode::Ideal()

[Roman Kennke]

See $SUBJECT. Reduces our upstream exposure. I will RFR the
BSC2::ideal_node() API to upstream shortly.

http://cr.openjdk.java.net/~rkennke/cmppnode-ideal/webrev.00/

Testing: tier3_gc_shenandoah

Ok?

Roman