Note [Arity for non-recursive join bindings]

Note [Arity for recursive join bindings] deals with recursive join
bindings. But what about /non-recursive/ones?  If we just call
findRhsArity, it will call arityType.  And that can be expensive when
we have deeply nested join points:
  join j1 x1 = join j2 x2 = join j3 x3 = blah3
                            in blah2
               in blah1
(e.g. test T18698b).

So we call cheapArityType instead.  It's good enough for practical
purposes.

(Side note: maybe we should use cheapArity for the RHS of let bindings
in the main arityType function.)