RFR: Use/sort (cached) RegionData not ShenandoahHeapRegionSet in partial heuristics

[Roman Kennke]

As suggested by Aleksey in the partial-traversal thread, here is the
first step of removing ShHRS: rip out the sorting stuff, and make the
partial heuristics use the cached RegionData instead.

http://cr.openjdk.java.net/~rkennke/partial-regiondata/webrev.00/

Testing: hotspot_gc_shenandoah

Ok?

Roman