Unpack TCO loops

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).

Start by reading the TCO pass and its existing handling of mutually recursive TCO bindings, then compare that with call-pattern-specialization. Done means known constructor arguments in TCO loops avoid allocations, without creating one specialized binding per constructor specialization; consider whether the optimization can also apply when the backend does not require TCO.

Written by the indexing model from the issue text.