Note [arityType for non-recursive let-bindings]

For non-recursive let-bindings, we just get the arityType of the RHS,
and extend the environment.  That works nicely for things like this
(#18793):
  go = \ ds. case ds_a2CF of {
               []     -> id
               : y ys -> case y of { GHC.Types.I# x ->
                         let acc = go ys in
                         case x ># 42# of {
                            __DEFAULT -> acc
                            1# -> \x1. acc (negate x2)

Here we want to get a good arity for `acc`, based on the ArityType
of `go`.

All this is particularly important for join points. Consider this (#18328)

  f x = join j y = case y of
                      True -> \a. blah
                      False -> \b. blah
        in case x of
              A -> j True
              B -> \c. blah
              C -> j False

and suppose the join point is too big to inline.  Now, what is the
arity of f?  If we inlined the join point, we'd definitely say "arity
2" because we are prepared to push case-scrutinisation inside a
lambda. It's important that we extend the envt with j's ArityType, so
that we can use that information in the A/C branch of the case.