gc tweaks

[Matthew Fluet]

> I'm missing something.  Isn't it clearly a maximum?  I guess that refs are a
> problem.  If you define share to not follow refs at all then it seems clear to
> me that it is a maximum.  You will only have one copy of all reachable objects.

You're right.  I was just remembering a comment Steve has in the CSE pass,
which notes that failing to have a total ordering on variables, he
partially orders them by hashes, which might miss a few cse's.  But, when
you're dealing with pointer equality, you're fine.