[m-dev.] diff: more bug fixes for liveness.

[Fergus Henderson]

Tyson Richard DOWD, you wrote:
> 
> I've added some documentation to this change, describing the invariant
> that accurate GC needs, and how it is achieved.
> 
> I'll commit this change very soon.
> 
> ===================================================================
> 
> Fix several bugs in liveness for accurate GC. 

I haven't given this change a really detailed examination, but it looks OK.

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