RFR: Use size_t consistently for region sizes, counts

[Aleksey Shipilev]

Hi,

Compilers complain how we use uint and size_t interchangeably for region sizes,
counts, and math involving them. This yields everything to size_t:
  http://cr.openjdk.java.net/~shade/shenandoah/region-idx-size_t/webrev.01/

Going to backport this to 8u later.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey