Note [Arity trimming]

Invariant (1) of Note [Arity invariants for bindings] is upheld by findRhsArity,
which calls trimArityType to trim the ArityType to match the Arity of the
binding.  Failing to do so, and hence breaking invariant (1) led to #5441.

How to trim?  If we end in topDiv, it's easy.  But we must take great care with
dead ends (i.e. botDiv). Suppose the expression was (\x y. error "urk"),
we'll get \??.⊥.  We absolutely must not trim that to \?.⊥, because that
claims that ((\x y. error "urk") |> co) diverges when given one argument,
which it absolutely does not. And Bad Things happen if we think something
returns bottom when it doesn't (#16066).

So, if we need to trim a dead-ending arity type, switch (conservatively) to
topDiv.

Historical note: long ago, we unconditionally switched to topDiv when we
encountered a cast, but that is far too conservative: see #5475