Minus sign in literal patterns is deleted sometimes

[amesgen]

While working on #694, I noted that the minus sign in (negative) literal patterns is deleted sometimes. With Ormolu 0.1.4.1 (--unsafe necessary):

foo = \case - 1 -> 1

to

foo = \case 1 -> 1

and

{-# LANGUAGE NoNegativeLiterals #-}

foo = \case -1 -> 1

to

{-# LANGUAGE NoNegativeLiterals #-}

foo = \case 1 -> 1

This is due to us ignoring the third field of NPat.