CRR (S/M): 7092309: G1: introduce old region set

[Tony Printezis]

Hi all,

This change adds to G1 the facility to keep track of the allocated old 
regions with the HeapRegionSet abstraction and moves us one step closer 
to having all regions being tracked by the same abstraction (after this, 
only the survivor / eden regions remain). I decided to work on the old 
region set now as it was a relatively easy change to implement and will 
make some follow-on CRs easier to implement. The webrev is here:

http://cr.openjdk.java.net/~tonyp/7092309/webrev.0/

Most of the changes were straightforward (I largely followed how we 
maintain the humongous region set). The only part which I had to 
slightly revamp was the code that tears down / rebuilds the region sets. 
This small refactoring, though, was worthwhile as it makes that code 
easier to extend in the future (when we introduce the eden / survivor 
lists).

Tony