RFR: Cherry-pick JDK-8213381: Hook to allow GC to inject Node::Ideal() calls plus Shenandoah parts

[Roland Westrelin]

> Roland: I know you said you'd try to upstream some of those parts. I
> think it's more useful (for now) to not need to discuss any of this
> upstream at all, and focus on the remaining stuff. It doesn't prevent
> upstreaming to shared code later, if it's truly useful. Ok?

Ok.

> http://cr.openjdk.java.net/~rkennke/JDK-8213381-shenandoah/webrev.00/

That looks good.

Roland.