RFR: 9+8u: Collection policy cleanups

[Aleksey Shipilev]

Hi,

This fixes a few compiler warnings that break MSVC build. First, we cannot
stack-alloc RegionGarbage array -- we have fixed it in one place before, but not
in the other one. Then, there are some int<->size_t conversions.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/collection-policy-cleanup/webrev.01/

This affects both 8u and 9.

Testing: hotspot_gc_shenandoah with 9 and 8u.

Thanks,
-Aleksey