RFR (S) 8242075: Shenandoah: rename ShenandoahHeapRegionSize flag

[Roman Kennke]

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8242075
> 
> There are ShenandoahMinRegionSize and ShenandoahMaxRegionSize flags. Having
> Shenandoah*Heap*RegionSize that overrides them is not very clear. Let's rename it to
> ShenandoahRegionSize.

Indeed. I have also stumbled over this a couple of times.

> Webrev:
>  https://cr.openjdk.java.net/~shade/8242075/webrev.01/
> 
> Testing: hotspot_gc_shenandoah

The patch looks good, thank you!

Roman