RFR (S) 8243573: Shenandoah: rename GCParPhases and related code

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8243573

ShenandoahPhaseTimings::GCParPhases is awkward: it is plural and it carries unnecessary "GC" prefix.
Should be just ShenandoahPhaseTimings::ParPhase. This also matches the local variables "par_phase"
to be used to store it, in contrast with just "phase" for ShenandoahPhaseTimings::Phase.

Webrev:
  https://cr.openjdk.java.net/~shade/8243573/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey