[BUG] Miscomputation of mutually recursive functions in loop

[vouillon]

Mutually recursive functions occurring in a loop that do not explicitly calls one-another are miscompiled.

let app f x = f x
let () =
  for i = 1 to 4 do
    let rec f x = if x = 0 then 1 else i * app g (x - 1)
    and g x = app f x
    in
    Format.eprintf "%d@." (g i)
  done

I get the following error when trying to run the generated code:``

TypeError: Cannot read property 'length' of undefined
    at caml_call1 (/tmp/a.js:2416:16)
    at app (/tmp/a.js:5655:36)
    at f (/tmp/a.js:5660:55)
    at caml_call1 (/tmp/a.js:2416:28)
    at app (/tmp/a.js:5655:36)
    at g (/tmp/a.js:5663:47)
    at /tmp/a.js:5665:15

Using the information from mutated_vars in group_closures instead of ntc seems to fix the issue. But that might not be enough to address #1007?

        let calls =
          if tc_only
          then calls
          else Var.Set.union calls (Addr.Map.find (fst x.cont) mutated_vars)
        in

[hhugo]

#1009 added regression tests for #1007. Your proposed change looks good to me, implemented in #1321