oprhNamesOfFamInst should include the type family itself (fix Trac #8469)

Trivial fix; matches oprhNamesOfClsInst.
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Refactor the topNormaliseNewType story, fixing Trac #8467

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
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