Describe the bug
The snippet

{-# LANGUAGE Arrows #-}

foo =
  proc x -> do
    returnA -< x
   1123

is formatted (with --unsafe) into

{-# LANGUAGE Arrows #-}

foo =
  proc x ->
    do
      returnA -< x 1123

with different AST.

This is basically an example for what @mrkkrp suspected in #719.

I found this while testing #707; I have no idea if anyone would ever write code like that.

Environment
current master