Arity: Should propagate arity type of scrutinee to case binder

Currently in GHC.Core.Opt.Arity we have

arityType env (Case scrut bndr _ alts)
  ...
  where
    env' = delInScope env bndr

Instead of defaulting to using the old idArity bndr, we should propagate the arity type from the scrutinee here. For an added bonus, we can reflect evaluation of the outermost Cost of that arity type to IsCheap.