A folow-up to D2521 (which addressed #12583), where the `Outputable` `ppr`
output was tweaked to display a list comprehension with only one `Stmt` as
`[Foo]` instead of `[Foo|]` (which isn't valid Haskell). I forgot to update
the corresponding code in `Language.Haskell.TH.Ppr` which pretty-prints
`CompE`, however, so this commit takes care of that.