Note [Care with unlifted bindings]

Consider (#22998)
    f x = let x::ByteArray# = <some literal>
              n::Natural    = NB x
          in wombat @192827 (n |> co)
where
  co :: Natural ~ KnownNat 192827
  wombat :: forall (n:Nat). KnownNat n => blah

Left to itself, the specialiser would float the bindings for `x` and `n` to top
level, so we can specialise `wombat`.  But we can't have a top-level ByteArray#
(see Note [Core letrec invariant] in GHC.Core).  Boo.

This is pretty exotic, so we take a simple way out: in specBind (the NonRec
case) do not float the binding itself unless it satisfies exprIsTopLevelBindable.
This is conservative: maybe the RHS of `x` has a free var that would stop it
floating to top level anyway; but that is hard to spot (since we don't know what
the non-top-level in-scope binders are) and rare (since the binding must satisfy
Note [Core let-can-float invariant] in GHC.Core).