RFR: JDK-8213615: GC/C2 abstraction for escape analysis

[Roman Kennke]

There's plenty of GC related+specific (G1, ZGC, Shenandoah) code in
escape.cpp that needs to be abstracted out into BarrierSetC2 or similar.

Consider for example the mess we needed to make in Shenandoah:
https://builds.shipilev.net/patch-openjdk-shenandoah-jdk-only-shared/b64-vs-dc1f9dec2018/src/hotspot/share/opto/escape.cpp.sdiff.html


The following proposed changeset covers all needs of G1, ZGC and Shenandoah:
http://cr.openjdk.java.net/~rkennke/JDK-8213615/webrev.01/

Some notes:
- Similar to how it works with the hooks for
Compile::final_graph_reshaping(), if GC returns true (meaning the GC
completely handled the current node), the main switch is skipped.
- The bodies of the unsafe (CAS, etc) handlers are factored out so that
they can be called back from GC handlers. For example, in Shenandoah we
have our own set of CAS nodes that need to call back into those.
- A bunch of methods in ConnectionGraph (e.g. add_local_var_and_edge)
needed to be made public so that they can be accessed from the GC code.
- ConnectionGraph::record_for_optimizer() body has been moved into
escape.cpp because it depends on PhaseIterGVN to be known, and I did not
feel like adding include phaseX.hpp to escape.hpp. This popped up
because of changed include order.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8213615

Testing: passes hotspot/jtreg:tier1 locally.

Thoughts? Reviews?

Roman

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