More improvements to worker/wrapper

This patch fixes #21888, and simplifies finaliseArgBoxities
by eliminating the (recently introduced) data type FinalDecision.

A delicate interaction meant that this patch
   commit d1c25a48
   Date:   Tue Jul 12 16:33:46 2022 +0100
   Refactor wantToUnboxArg a bit

make worker/wrapper go into an infinite loop.  This patch
fixes it by narrowing the handling of case (B) of
Note [Boxity for bottoming functions], to deal only the
arguemnts that are type variables.  Only then do we drop
the trimBoxity call, which is what caused the bug.

I also
* Added documentation of case (B), which was previously
  completely un-mentioned.  And a regression test,
  T21888a, to test it.

* Made unboxDeeplyDmd stop at lazy demands.  It's rare anyway
  for a bottoming function to have a lazy argument (mainly when
  the data type is recursive and then we don't want to unbox
  deeply).  Plus there is Note [No lazy, Unboxed demands in
  demand signature]

* Refactored the Case equation for dmdAnal a bit, to do less
  redundant pattern matching.