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

[Roland Westrelin]

> 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.

Both changes make some sense as non GC specific changes. So it seems to
me they belong in shared code. We can always fall back to making them
shenandoah specific if they are not accepted.

ROland.