Optimise nullary type constructor usage

During the compilation of programs GHC very frequently deals with the Type type, which is a synonym of TYPE 'LiftedRep. This patch teaches GHC to avoid expanding the Type synonym (and other nullary type synonyms) during type comparisons, saving a good amount of work. This optimisation is described in Note [Comparing nullary type synonyms].

To maximize the impact of this optimisation, we introduce a few special-cases to reduce TYPE 'LiftedRep to Type. See Note [Prefer Type over TYPE 'LiftedPtrRep].

Closes #17958 (closed).