G1: Consolidate per-region is-humongous query in G1CollectedHeap

In `G1CollectedHeap`, `_humongous_reclaim_candidates` is used to denote whether a particular humongous region can be earge-reclaimed. Meanwhile, `_region_attr` is also used for quick per-region info query.

There's no strong reason why the former needs a dedicated table. Try to merge the two.