Fix arityType: -fpedantic-bottoms, join points, etc

This MR fixes #21694 and #21755

* For #21694 the underlying problem was that we were calling arityType
  on an expression that had free join points.  This is a Bad Bad Idea.
  See Note [No free join points in arityType].

* I also made andArityType work correctly with -fpedantic-bottoms;
  see Note [Combining case branches: andWithTail].

* I realised that, now we have ae_sigs giving the ArityType for
  let-bound Ids, we don't need the (pre-dating) special code in
  arityType for join points. But instead we need to extend the env for
  Rec bindings, which weren't doing before.  More uniform now.  See
  Note [arityType for let-bindings].

  This meant we could get rid of ae_joins, and in fact get rid of
  EtaExpandArity altogether.  Simpler.

  And finally, it was the strange treatment of join-point Ids (involving
  a fake ABot type) that led to a serious bug: #21755.  Fixed by this
  refactoring

* Rewrote Note [Combining case branches: optimistic one-shot-ness]

Compile time improves slightly on average:

Metrics: compile_time/bytes allocated
---------------------------------------------------------------------------------------
CoOpt_Read(normal) ghc/alloc    803,788,056    747,832,680  -7.1% GOOD
    T18223(normal) ghc/alloc    928,207,320    959,424,016  +3.1%  BAD
         geo. mean                                          -0.3%
         minimum                                            -7.1%
         maximum                                            +3.1%

On Windows it's a bit better: geo mean is -0.6%, and three more
benchmarks trip their compile-time bytes-allocated threshold (they
were all close on the other build):

   T18698b(normal) ghc/alloc    235,619,776    233,219,008  -1.0% GOOD
     T6048(optasm) ghc/alloc    112,208,192    109,704,936  -2.2% GOOD
    T18140(normal) ghc/alloc     85,064,192     83,168,360  -2.2% GOOD

I had a quick look at T18223 but it is knee deep in coercions and
the size of everything looks similar before and after.  I decided
to accept that 3.4% increase in exchange for goodness elsewhere.

Metric Decrease:
    CoOpt_Read
    T18140
    T18698b
    T6048

Metric Increase:
    T18223

(cherry picked from commit 5e282da37e19a1ab24ae167daf32276a64ed2842)