RFR: Cherry-pick: 8213489: GC/C2 abstraction for Compile::final_graph_reshaping() plus Shenandoah parts

[Roman Kennke]

In preparation and to ease next upstream merge, I'd like to cherry-pick
"8213489: GC/C2 abstraction for Compile::final_graph_reshaping()" and
the corresponding Shenandoah parts.

This is a particularily nasty one to merge because it doesn't actually
conflict and would compile just fine, yet leave all the Shenandoah parts
in shared code.

http://cr.openjdk.java.net/~rkennke/JDK-8213489-shenandoah/webrev.00/

Passes tier3_gc_shenandoah

OK?

Roman