Unpack TCO loops

[natefaubion]

It would be nice to unpack TCO loops, such that TCO arguments that are always known constructors don't need allocations. This would give us something comparable to call-pattern-specialization in the TCO pass. We essentially already do this for mutually recursive TCO bindings, as a special case.

It doesn't strictly need to be a TCO pass, as it would obviously be nice to have this even when the backend doesn't need TCO. It just seems like a straightforward extension for now.

Additionally, by doing it as part of codegen, we can avoid an explosion of specialized bindings (one for every specialization of constructors).