[tree-ssa] Avoid nondeterminism in tree-ssanames

[Daniel Berlin]

>>   And will there or will there
>> not be a free list mainatined within the zone?
>
> Not within the zone collector, only after collecting objects (doh!).  
> You need
> to maintain your own objects cache.

Well, you could make it free specific objects without any trouble prior 
to marking or collection, it just "doesn't" right now. :P

I can make it able to do so if you like.