Fix loopjitting

Improve loop arg/ret discovery

Loop-jitting was incorrectly turning local variables into arguments of lifted loop functions when a variable with the same name is used outside of the loop.  This patch improves the overall argument and return value discovery to properly consider the read/write order of variables.  

Do you need last_use_in_loop? It should be enough to know the loop boundaries inside the bytecode, or am I missing something?