[haskell-llvm] enforce block structure of functions

[Henning Thielemann]

In LLVM.Core.Instructions I read:

-- TODO:
-- Use Terminate to ensure bb termination (how?)


To answer this question: I would introduce a Block monad. LLVM assembly 
instructions become actions of the Block monad and a block must be 
terminated with a terminator directive like 'br' or 'ret'. This rule is 
ensured by a function that lifts a Block to a CodeGenFunction:
   embedBlock :: Block r Terminate -> CodeGenFunction r ()
  Since we recently started to use the monadic result () for instructions 
without a result like 'store', we can no longer use the type synonym 
Terminate = (). But we could add a new type (data Terminate = Terminate) 
or just (data Terminate).

This change would break most of the code that uses the llvm package, but 
it would document and force the overall structure of a piece of LLVM 
assembly code written in Haskell.

As a light proposal for now, I would just like to define a new type 
Terminate, that is distinct from ().