`G1FullGCHeapRegionAttr` is used to query per-region info during a full GC. Some potential cleanups for it:

1. The term "NotCompacted" may impose the impression that it is the complement of the "compacted" set; IOW, the interpretation could be that a region is either compacted or not compacted. However, the semantics is actually that we "skip compaction" for those regions. Possibly "SkipCompacting" is less ambiguous.

2. In `G1CalculatePointersClosure`, instead of updating the table for all regions, only updating those that actually require updating (freed regions, skipped regions, etc).