Simon Peyton Jones authored May 04, 2021 and Marge Bot committed May 11, 2021

In GHC.Core.Opt.SpecConstr.spec_one we were giving join-points an
incorrect join-arity -- this was fallout from

    commit c71b2204
    Author: Simon Peyton Jones <simonpj@microsoft.com>
    Date:   Thu Apr 8 23:36:24 2021 +0100

        Improvements in SpecConstr

        * Allow under-saturated calls to specialise
          See Note [SpecConstr call patterns]
          This just allows a bit more specialisation to take place.

and showed up in #19780. I refactored the code to make the new
function calcSpecInfo which treats join points separately.

In doing this I discovered two other small bugs:

* In the Var case of argToPat we were treating UnkOcc as
  uninteresting, but (by omission) NoOcc as interesting. As a
  result we were generating SpecConstr specialisations for functions
  with unused arguments.  But the absence anlyser does that much
  better; doing it here just generates more code.  Easily fixed.

* The lifted/unlifted test in GHC.Core.Opt.WorkWrap.Utils.mkWorkerArgs
  was back to front (#19794).  Easily fixed.

* In the same function, mkWorkerArgs, we were adding an extra argument
  nullary join points, which isn't necessary.  I added a test for
  this.  That in turn meant I had to remove an ASSERT in
  CoreToStg.mkStgRhs for nullary join points, which was always bogus
  but now trips; I added a comment to explain.