Note [idArity varies independently of dmdTypeDepth]

In general, an Id `f` has two independently varying attributes:

* f's idArity, and
* the dmdTypeDepth of f's demand signature

For example, if f's demand signature is <L><L>, f's arity could be
greater than, or less than 2. Why?  Because both are conservative
approximations:

* Arity n means "does no expensive work until applied to at least n args"
  (e.g. (f x1..xm) is cheap to bring to HNF for m<n)

* Dmd sig with n args means "here is how to transform the incoming demand
  when applied to n args".  This is /semantic/ property, unrelated to
  arity. See GHC.Types.Demand Note [Understanding DmdType and DmdSig]

We used to check in GHC.Core.Lint that dmdTypeDepth <= idArity for a let-bound
identifier. But that means we would have to zap demand signatures every time we
reset or decrease arity.

For example, consider the following expression:

    (let go x y = `x` seq ... in go) |> co

`go` might have a strictness signature of `<1L><L>`. The simplifier will identify
`go` as a nullary join point through `joinPointBinding_maybe` and float the
coercion into the binding, leading to an arity decrease:

    join go = (\x y -> `x` seq ...) |> co in go

With the CoreLint check, we would have to zap `go`'s perfectly viable strictness
signature.

However, in the case of a /bottoming/ signature, f : <L><L>b, we /can/
say that f's arity is no greater than 2, because it'd be false to say
that f does no work when applied to 3 args.  Lint checks this constraint,
in `GHC.Core.Lint.lintLetBind`.