Sticky Immix is broken for `mmtk-support-moving-upstream`

[qinsoon]

I am trying to get sticky immix working on our moving branch mmtk-support-moving-upstream, and observed the following bug:

1. Create task X in the non-moving space
2. X is added to all_tasks.
3. Nursery GC happens.
4. X is not in the nursery space, and is not traced. X is not marked.
5. At the end of the GC, we sweep all_tasks by checking is_live() with MMTk. MMTk calls is_live() for the space. X is in the non-moving Immix space which decides liveness based on the mark. X is not marked, thus gets removed from all_tasks.
6. New roots are pushed to X's shadow stack with no write barrier. So X is not in the mod buffer either.
7. Nursery GC happens.
8. X is not in all_tasks. We won't scan its shadow stack and the new roots will not be scanned properly.
9. X is not in the nursery space, nor the mod buffer. So it not traced or scanned.
10. New roots become dead.

I think the issue is that is_live() should not return false for X whose liveness is actually unknown. See mmtk/mmtk-core#1422