Fix a bug with `$it` variable decompilation

Clarification and motivation

In #1772 (closed) we started to decompile $it variable (the last computed value). But it introduced a new bug.

Take this contract

type 'a proxy = Proxy

let some_complex_variable =
  { a = 42
  ; b = "abc"
  ; c =
    { f1 = 1tz
    ; f2 = 28n
    ; f3 = (Proxy : unit proxy)
    }
  }

let main (_, s : unit * int) : operation list * int =
  let res = some_complex_variable.a in
  (([] : operation list), s + res)

The interesting part here is { f1 = 1tz; f2 = 28n; f3 = (Proxy : unit proxy) }. A compiled source for it looks like

{ /* File "test3.mligo", line 7, character 4 to line 10, character 5 */
               { /* File "test3.mligo", line 9, characters 12-17 */ UNIT } ;
               { /* File "test3.mligo", line 7, character 4 to line 10, character 5 */
                 { /* File "test3.mligo", line 8, characters 11-14 */ PUSH nat 28 } ;
                 { /* File "test3.mligo", line 7, characters 11-14 */ PUSH mutez 1000000 } ;
                 PAIR
                 /* File "test3.mligo", line 7, character 4 to line 10, character 5 */ } ;
               PAIR
               /* File "test3.mligo", line 7, character 4 to line 10, character 5 */ } ;

We're interested in /* File "test3.mligo", line 7, character 4 to line 10, character 5 */ location. With --michelson-format json it has a { f1 : mutez; f2 : nat; f3 : unit proxy } type for all locations. It means that if I try to decompile not the topmost value, I'll get a transpilation error.

As a result in the debugger, we'll see a halting $it: computing....

Acceptance criteria

• We don't have a halting $it: computing... in the variables pane.