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.
Edited by Sebastian Graf