As Note [Eta-expansion and join points] describes,
this patch makes arityType deal correctly with join points.
What was there before was not wrong, but yielded lower
arities than it could.

Fixes #18328

In base GHC this makes no difference to nofib.

        Program           Size    Allocs   Runtime   Elapsed  TotalMem
--------------------------------------------------------------------------------
         n-body          -0.1%     -0.1%     -1.2%     -1.1%      0.0%
--------------------------------------------------------------------------------
            Min          -0.1%     -0.1%    -55.0%    -56.5%      0.0%
            Max          -0.0%      0.0%    +16.1%    +13.4%      0.0%
 Geometric Mean          -0.0%     -0.0%    -30.1%    -31.0%     -0.0%

But it starts to make real difference when we land the change to the
way mkDupableAlts handles StrictArg, in fixing #13253 and friends.
I think this is because we then get more non-inlined join points.