For now these errors are handled via `@panic` rather than `unreachable`.
These are relatively likely bugs to occur at this early stage of
development, and handling them as panics lets us ship release builds
of the compiler without worrying about undefined behavior.

Furthermore, in stage1, `@panic` is implemented to include an error
return trace, while `unreachable` is not. In this case, the error return
traces are extremely helpful in debugging the compiler.