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

[Roman Kennke]

In preparation of next upstream merge, I'd like to cherry-pick
JDK-8213381, and move around all the Shenandoah related parts into
ShenandoahBarrierSetC2.

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?

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

Testing: tier3_gc_shenandoah

Ok?

Roman