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

[Roman Kennke]

Yes!

Thanks,
Roman

> 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}
>