Note [Not-necessarily-lifted join points]

A join point variable is essentially a goto-label: it is, for example,
never used as an argument to another function, and it is called only
in tail position. See Note [Join points] and Note [Invariants on join points],
both in GHC.Core. Because join points do not compile to true, red-blooded
variables (with, e.g., registers allocated to them), they are allowed
to be representation-polymorphic.
(See invariant #6 in Note [Invariants on join points] in GHC.Core.)

However, in this byte-code generator, join points *are* treated just as
ordinary variables. There is no check whether a binding is for a join point
or not; they are all treated uniformly. (Perhaps there is a missed optimization
opportunity here, but that is beyond the scope of my (Richard E's) Thursday.)

We thus must have *some* strategy for dealing with representation-polymorphic
and unlifted join points. Representation-polymorphic variables are generally
not allowed (though representation-polymorphic join points *are*; see
Note [Invariants on join points] in GHC.Core, point 6), and we don't wish to
evaluate unlifted join points eagerly.
The questionable join points are *not-necessarily-lifted join points*
(NNLJPs). (Not having such a strategy led to #16509, which panicked in the
isUnliftedType check in the AnnVar case of schemeE.) Here is the strategy:

1. Detect NNLJPs. This is done in isNNLJoinPoint.

2. When binding an NNLJP, add a `\ (_ :: (# #)) ->` to its RHS, and modify the
   type to tack on a `(# #) ->`.
   Note that functions are never representation-polymorphic, so this
   transformation changes an NNLJP to a non-representation-polymorphic
   join point. This is done in bcPrepSingleBind.

3. At an occurrence of an NNLJP, add an application to void# (called voidPrimId),
   being careful to note the new type of the NNLJP. This is done in the AnnVar
   case of schemeE, with help from protectNNLJoinPointId.

Here is an example. Suppose we have

  f = \(r :: RuntimeRep) (a :: TYPE r) (x :: T).
      join j :: a
           j = error @r @a "bloop"
      in case x of
           A -> j
           B -> j
           C -> error @r @a "blurp"

Our plan is to behave is if the code was

  f = \(r :: RuntimeRep) (a :: TYPE r) (x :: T).
      let j :: ((# #) -> a)
          j = \ _ -> error @r @a "bloop"
      in case x of
           A -> j void#
           B -> j void#
           C -> error @r @a "blurp"

It's a bit hacky, but it works well in practice and is local. I suspect the
Right Fix is to take advantage of join points as goto-labels.