RFR: Constify ShHeapRegionSet and ShCollectionSet

[Roman Kennke]

I've noticed that we have way too many derefs to get to cset's
_biased_cset_map in hot loops. Making all constant fields const in
ShCollectionSet helps a little. Also, I've seen it deref
ShenandoahHeapRegion::RegionSizeBytesShift in hot loop. Making this a
const field in the cset class helps this too.

I've also done the ShenandoahHeapRegion::RegionSizeBytesShift change in
ShHeapRegionSet. The other fields are already good there.

I've got more stuff in the pipe to improve the situation, but that's
less trivial to do.


http://cr.openjdk.java.net/~rkennke/constify-cset/webrev.00/

Testing: tier3_gc_shenandoah (ongoing, pushing when clean)

Ok?

Roman