Fix invariant in mkAppCoFlexible

mkAppCoFlexible was breaking the invariant that the head of a TyConAppCo cannot
be a type synonym.  This small patch fixes it.

(cherry picked from commit 21f17d06)

compiler/types/Coercion.lhs

@@ -899,7 +899,9 @@ mkAppCo co1 co2 = mkAppCoFlexible co1 Nominal co2
 mkAppCoFlexible :: Coercion -> Role -> Coercion -> Coercion
 mkAppCoFlexible (Refl r ty1) _ (Refl _ ty2)
   = Refl r (mkAppTy ty1 ty2)
-mkAppCoFlexible (Refl r (TyConApp tc tys)) r2 co2
+mkAppCoFlexible (Refl r ty1) r2 co2
+  | Just (tc, tys) <- splitTyConApp_maybe ty1
+    -- Expand type synonyms; a TyConAppCo can't have a type synonym (Trac #9102)
   = TyConAppCo r tc (zip_roles (tyConRolesX r tc) tys)
   where
     zip_roles (r1:_)  []        = [maybeSubCo2 r1 r2 co2]