Undecidable instances slip through

Consider

{-# LANGUAGE FlexibleInstances, TypeFamilies, ConstraintKinds #-}

type family F a :: Constraint

class C a where
instance (F a) => C [[a]] where

This should be rejected because the F a constraint could expand to be arbitrarily large, depending on F.

But it's accepted by HEAD. There's simply a missing check in checkInstTermination.