This PR correctly populates the `xType` field of the `IR.FnBody.case`
constructor. It turns out that there is no obvious consequence for this
being incorrect, because it is conservatively recomputed by the `Boxing`
pass.
This currently relies on the encoding pun of Nat.zero as the first
tagged constructor of Nat. Since Nat.succ is lowered to addition, it
makes sense to also lower Nat.zero to a zero literal. This might also
expose more optimization opportunities in the future.
This is part 2 of 2 of #4801 (which closes#4654). That PR was split in
two to allow a stage0 update between declaring the `usize` functions and
using them where they are needed.
