once, local-refs, and threads

[Matthew Fluet]

> It's the quadratic (in the number of
> ThreadCopyPoints) search through the pairs of ThreadCopyPoints that
> bothers me.  Thread_saves aren't so bad, but the main function in a self
> compile is going to have tons of non-tail calls. 

Please excuse my earlier brain-fry; two simple bool refs on localizable
refs that indicates whether or not we crossed a ThreadCopyPoint when
backpropagating Ref_assign uses and Ref_deref uses suffices.  If both
bools are true at the end, the ref is not localizable.