RFR: Cherry-pick: 8214055: GC/C2 abstraction for phaseX

[Aleksey Shipilev]

On 11/22/18 9:07 PM, Roman Kennke wrote:
> In order to ease the next upstream merge, and because the patch does not
> even conflict, I'd like to cherry-pick: 8214055: GC/C2 abstraction for
> phaseX and move/add the corresponding Shenandoah parts.
> 
> http://cr.openjdk.java.net/~rkennke/JDK-8214055-shenandoah/webrev.01/

*) Odd newline at L1494 in shenandoahBarrierSetC2.cpp:

1493   }
1494
1495 }

Otherwise good.

-Aleksey