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

[Roland Westrelin]

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

I don't think that's needed. We can remove the if (UseShenandoahGC) {
check and upstream that code. I intended to do that soon.

Roland.