Note [Join points]

We check the rules listed in Note [Invariants on join points] in GHC.Core. The
only one that causes any difficulty is the first: All occurrences must be tail
calls. To this end, along with the in-scope set, we remember in le_joins the
subset of in-scope Ids that are valid join ids. For example:

  join j x = ... in
  case e of
    A -> jump j y -- good
    B -> case (jump j z) of -- BAD
           C -> join h = jump j w in ... -- good
           D -> let x = jump j v in ... -- BAD

A join point remains valid in case branches, so when checking the A
branch, j is still valid. When we check the scrutinee of the inner
case, however, we set le_joins to empty, and catch the
error. Similarly, join points can occur free in RHSes of other join
points but not the RHSes of value bindings (thunks and functions).