Idempotence bug with comments in data declaration

Input:
```
data A =
     B  -- C

   -- D
   | E
```

Output:

```
Formatting is not idempotent:
  before: " B -- C\n  | -- D\n   "
  after:  " B -- C\n    -- D\n  |"
Please, consider reporting the bug.
```

Investigating further, it formats it as:

```
data A
  = B -- C
  | -- D
    E
```

which then gets reformatted as:

```
data A
  = B -- C
    -- D
  | E
```