A bit of a mess had accumulated, with unclear invariants.

* Remove splitNewTypeRepCo_maybe, in favour of topNormaliseNewType_maybe
  (which had the same signature but behaved subtly differently).

* Make topNormaliseNewType_maybe guaranteed to return a non-newtype
  if it says (Just ty).  This is what was causing the loop in #8467

* Apply similar tidying up to FamInstEnv.topNormaliseType