Note [TailCallInfo]

The occurrence analyser determines what can be made into a join point, but it
doesn't change the binder into a JoinId because then it would be inconsistent
with the occurrences. Thus it's left to the simplifier (or to simpleOptExpr) to
change the IdDetails.

The AlwaysTailCalled marker actually means slightly more than simply that the
function is always tail-called. See Note [Invariants on join points].

This info is quite fragile and should not be relied upon unless the occurrence
analyser has *just* run. Use 'Id.idJoinPointHood' for the permanent state of
the join-point-hood of a binder; a join id itself will not be marked
AlwaysTailCalled.

Note that there is a 'TailCallInfo' on a 'ManyOccs' value. One might expect that
being tail-called would mean that the variable could only appear once per branch
(thus getting a `OneOcc { }` occurrence info), but a join
point can also be invoked from other join points, not just from case branches:

  let j1 x = ...
      j2 y = ... j1 z {- tail call