check that every block ends with a terminator

[mnikander]

• defined the set of terminators
• implemented function to check that every block ends in a terminator
• every call to evaluate includes this check (similar to the single-assignment check)

[mnikander]

Terminators can be:

• branch
• jump
• return
• exit

'call' is a bit problematic, since it doesn't usually terminate a block -- unless you are doing continuation-passing style (CPS). I could say YAGNI, and if I ever do need it I can always introduce a separate instruction for calling a continuation. For a true CPS interpreter I'd probably want to change how the stack grows though. I'd need the tail-call optimization for sure, or an explicit tail-call operation.

[mnikander]

See https://llvm.org/docs/LangRef.html for the list of terminator instructions in LLVM IR