8048112: G1 Full GC needs to support the case when the very first region is not available

Refactor preparation for compaction during Full GC so that it lazily initializes the first compaction point. This also avoids problems later when the first region may not be committed. Also reviewed by K. Barrett.

Reviewed-by: brutisso

hotspot/src/share/vm/gc_implementation/g1/heapRegionSet.hpp

@@ -119,7 +119,7 @@ protected:
 public:
   const char* name() { return _name; }
 
-  uint length() { return _count.length(); }
+  uint length() const { return _count.length(); }
 
   bool is_empty() { return _count.length() == 0; }