Simon Peyton Jones authored Jul 14, 2020 and Marge Bot committed Jul 18, 2020

This MR (for #18449) refactors the Simplifier's treatment
of join-point binders.

Specifically, it puts together, into
     GHC.Core.Opt.Simplify.Env.adjustJoinPointType
two currently-separate ways in which we adjust the type of
a join point. As the comment says:

-- (adjustJoinPointType mult new_res_ty join_id) does two things:
--
--   1. Set the return type of the join_id to new_res_ty
--      See Note [Return type for join points]
--
--   2. Adjust the multiplicity of arrows in join_id's type, as
--      directed by 'mult'. See Note [Scaling join point arguments]

I think this actually fixes a latent bug, by ensuring that the
seIdSubst and seInScope have the right multiplicity on the type
of join points.

I did some tidying up while I was at it.  No more
setJoinResTy, or modifyJoinResTy: instead it's done locally in
Simplify.Env.adjustJoinPointType