RFR: JDK-8213381: Hook to allow GC to inject Node::Ideal() calls

[Roman Kennke]

Shenandoah injects a couple of reshapings into some implementations of
Node::Ideal() (namely CallLeafNode::Ideal() and CmpPNode::Ideal()). I
propose to provide a hook into BarrierSetC2 to better abstract this.
This might be useful for other GCs too.

The approach that I've chosen here is to call BSC2::ideal_node(..) from
the places in phaseX.cpp where we also call Node::Ideal(..). I've
introduced a helper method to do this: first call BSC2::ideal_node(..),
if that returns NULL, call into the usual Node::Ideal(..).

BarrierSetC2::ideal_node(..) must follow the same contract as
Node::Ideal(..) of course.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8213381
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8213381/webrev.00/

Testing: hotspot/jtreg:tier1 passes locally.

Can I please get reviews?

Roman

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