dillon1000/react

Exhaustive switches for some terminal handling

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.