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

[Roman Kennke]

Should I take this upstreaming? I've already everything lined up:

https://bugs.openjdk.java.net/browse/JDK-8213381

Roman

>> 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.
>