check that we correctly sign-extend values that we write

[jvanburen]

          Not specific to this PR, but I noticed this does not write sign-extended int32#s into mixed blocks. Flambda2 tries to make sure the full content of blocks is deterministic (e.g. [in closures](https://github.com/ocaml-flambda/flambda-backend/blob/main/middle_end/flambda2/to_cmm/to_cmm_shared.ml#L398)), so I think [this code](https://github.com/ocaml-flambda/flambda-backend/blob/main/middle_end/flambda2/to_cmm/to_cmm_primitive.ml#L188) should probably be changed. But maybe this can never be used on static blocks so we don't care---we should check with the flambda2 devs.

Originally posted by @TheNumbat in #3404 (comment)