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

[Aleksey Shipilev]

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

-- 
Thanks,
-Aleksey