Consider this note (TcTyClsDecls)

  Note [Type-checking type patterns]
  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  When typechecking the patterns of a family instance declaration, we can't
  rely on using the family TyCon itself, because this is sometimes called
  from within a type-checking knot. (Specifically for closed type families.)
  The FamTyConShape gives just enough information to do the job.

I realised that this exact purpose can be served by TcTyCons, and
in fact rather better.  So this patch

* Refactors FamTyConShape out of existence, replacing it with TcTyCOn

* I also got rid Type.filterOutInvisibleTyVars, which was a very
  complex way to do something quite simple.  I replaced the calls
  with TyCon.tyConVisibleTyVars.

No change in behaviour.