RFR(XS) Eliminating _num_regions variable in ShenandoahHeap

[Zhengyu Gu]

Eliminate dual states which might result out-of-sync problem.
and use _ordered_regions->active_regions() solely.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/heap_regions_count/webrev.00/


Test:
   hotspot_gc_shenandoah: fastdebug and release


Thanks,

-Zhengyu