This minor refactoring re-attaches Note [Add unfolding for scrutinee].
It had become detached, which led me on a bit of a wild goose
chase.

While I was at it, I made the code work right for the case where
the scrutinee is of form (x |> co); I don't think this is an important
improvement.

I also make simplAlt unconditionally zap occurrence information on
case-alternative binders (see Note [Case alternative occ info]);
it was almost always being zapped and the additional complexity seems
not worth it.