Dead code elimination of some unused references

[vouillon]

If we have some code like the following where f and g are not used otherwise, neither f nor g have to be considered live.

let f = ref None
....
let () = f := Some g

More generally, an assignment Set_field (x, _, y) will not mark x as live and will mark y as live only if x is live.

[hhugo]

x-ref ocaml-community/yojson#168 which is working around the current limitation.