RFR (S) 8242227: Shenandoah: transit regions to cset state when adding to collection set

[Roman Kennke]

Yes, looks good!

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8242227
> 
> Current code has awkward ShenandoahCollectionSet method that is called after cset selection. It
> walks the regions again and transits the regions to cset status. At this point, we can just do this
> during SHCSet::add_region() and save a walk.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8242227/webrev.01/
> 
> Testing: hotspot_gc_shenandoah, benchmarks
>