Updates two points in the compiler that were easy to miss when adding new
terminals:
* HIRBuilder's `removeUnreachableFallthroughs()` nulls out unreachable
fallthroughs, but this had a non-exhaustive `if` statement. It now uses a helper
function which internally has an exhaustive switch.
* LeaveSSA needs to schedule block fallthroughs, but had a non-exhaustive `if`
statement. It also uses a helper function which internally has an exhaustive
switch.
cc @poteto since you ran into this (ie the compiler not alerting you to update
these places) w your diffs.