RFR (XS) 8242083: Shenandoah: split "Prepare Evacuation" tracking into cset/freeset counters

[Roman Kennke]

Ok.

Roman


> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8242083
> 
> Need this to follow up on performance behaviors with lots of regions better.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8242083/webrev.01/
> 
> Logs before/after:
>   https://cr.openjdk.java.net/~shade/8242083/before.txt
>   https://cr.openjdk.java.net/~shade/8242083/after.txt
> 
> Testing: hotspot_gc_shenandoah, eyeballing gc logs
>