G1: Improve liveness check for empty pinned regions after full gc marking

For open archive regions after marking we check whether anything is still live in there using a bitmap scan.

However in JDK-8263495 we added code to gather per-region liveness during full gc marking, so that bitmap scan can be replaced by a much faster lookup of the current liveness in that region.

to be exact, in:

bool G1FullGCPrepareTask::G1CalculatePointersClosure::do_heap_region(HeapRegion* hr) {

    } else if (hr->is_open_archive()) {
      bool is_empty = _bitmap->get_next_marked_addr(hr->bottom(), hr->top()) >= hr->top();
      if (is_empty) {
        free_open_archive_region(hr);
      }

the is_empty assignment can be rewritten as proposed.