RFR (S) 8241844: Shenandoah: rename ShenandoahHeapRegion::region_number

[Roman Kennke]

> RFR:
>   https://bugs.openjdk.java.net/browse/JDK-8241844
> 
> ShenandoahHeapRegion::region_number is too verbose of the name, plus most of the code already treats
> it as "index" in the local variable. Can rename it for consistency. I am not sure whether it should
> be "idx" or "index".
> 
> Candidate fix:
>   https://cr.openjdk.java.net/~shade/8241844/webrev.01/
> 
> Testing: hotspot_gc_shenandoah

I'd prefer index, but if it's too much trouble to rename it, then leave
it as idx.

Roman