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

[Roman Kennke]

>> But the stuff in LoadNode::Identity() is Shenandoah-specific, or is it
>> not?
> 
> I don't think the changes in LoadNode::Identity() or CmpPNode::Ideal()
> are shenandoah specific. Maybe useless with ZGC but still I don't see
> how they can hurt it.

Ok.

However, with the abstraction for BSC2::ideal_node(..) that I proposed,
we can easily move our CmpPNode::Ideal(..) stuff into ShenandoahBSC2,
and avoid discussing that part upstream ;-) And Erik said he'd like such
an abstraction anyway, so why not.

Roman