Wrong alignment while adding placeholder for class instances

### Steps to reproduce

With the following code
```haskell
data A
instance Eq A where
```
Execute code action to add placeholder for `==` with signature

### Expected behaviour

```haskell
instance Eq A where
(==) :: A -> A -> Bool
(==) = _
```

### Actual behaviour

```haskell
instance Eq A where (==) :: A -> A -> Bool
(==) = _
```

---

It **only** happens while the instance definition is at the last line and there is no extra line later. It doesn't matter before we have `add instance signature code action` but will result in errors after #2920.