RFR: 8226823: Adjust BarrierSetC2 for C2 runtime calls

[Roman Kennke]

 In JDK-8226695 we want to emit runtime calls in C2 ideal graph that
return an oop. This requires some adjustments in the GC interface for
escape analysis in order to work.

Specifically, we need:
- Access to a bunch of stuff in ConnectionGraph that is currently
private. Unfortunately we can't just befriend BarrierSetC2 and
transitively its subclasses.
- Apply the GC hook to EA earlier such that it can also handle calls.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8226823
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8226823/webrev.00/

Can I please get reviews?

Thanks,
Roman