During full-gc, multiple (almost) full regions starting at the beginning of old-gen form the dense-prefix, which will not be compacted, i.e. objects on them are not moved.

The following are two special operations we perform on the dense prefix

1. fill_dead_objs_in_dense_prefix: create filler so that the dense-prefix is parseable -- one can perform object iteration.

2. verify_filler_in_dense_prefix: it's debug-only to check dead space is occupied by filler objects.

Since most regions in dense prefix are (almost) full, we can speed up both operations by skipping full regions.