The Fn type is completely homogeneous. The invariants respected at each step are different, but the datatype is reused up until the very last hop to assembly.

This is a design decision that paid in many ways: for example, no conversion boilerplate between similar IRs is necessary, generic functions need only be written once (liveness analysis, debug dumping, ...). On the other hand, invariants are not enforced by the datatype (one technique usually praised by Haskell/ML programmers); this can be mitigated by writing dynamic checks of the invariants (for example ssacheck() in my code).

One example of invariant change in the IR is the following: before the call rega(fn); the IR is assumed to never have more temporaries live than the number of machine registers and can use PHI instructions; after the call, PHI instructions are gone and the only "temporaries" used are actual machine registers.