RFR (S) 8242212: Shenandoah: initialize ShenandoahHeuristics::_region_data eagerly

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8242212

This is the leftover from the time Shenandoah heap sizing policy drove the dynamic number of
regions. Now Shenandoah always have the same number of regions, so we can initialize _region_data
eagerly and dodge the hiccup on the very first GC cycle.

Fix:
  https://cr.openjdk.java.net/~shade/8242212/webrev.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}

-- 
Thanks,
-Aleksey