The element type of `List` maybe a type family instacen, rather than a
trivial type.
For example in Trac #14547,

```
{-# LANGUAGE TypeFamilies, OverloadedLists #-}

class Foo f where
        type It f
        foo :: [It f] -> f

data List a = Empty | a :! List a deriving Show

instance Foo (List a) where
        type It (List a) = a
        foo [] = Empty
        foo (x : xs) = x :! foo xs
```

Here the element type of `[]` is `It (List a)`, we should also normalize
it as `a`.

Test Plan: make test TEST="T14547"

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #14547

Differential Revision: https://phabricator.haskell.org/D4624