Wrong alignment while adding placeholder for class instances

[July541]

Steps to reproduce

With the following code

data A
instance Eq A where

Execute code action to add placeholder for == with signature

Expected behaviour

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

Actual behaviour

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.

[July541]

This is not easy to write a test because we'll insert a new empty line for every file.