Prevent fatal compiler error with unboxing code and GADTs

[Ekdohibs]

This prevents a crash on this example:

type foo =
  | A of float
  | B of int

type bar = Other

type 'a t =
  | Foo : foo -> foo t
  | Bar : bar t


let f (type a) : a t -> a = function
  | Foo x -> x
  | Bar -> Other


let baz p x z =
  let a =
    if p then (f x)
    else B z
  in
  Sys.opaque_identity a

I think we could also rewrite the code to Invalid in this case, but I'm not sure how to do that.

[chambart]

I tried to think about potential problems since this might be a bit tricky. But this seems sound.

It would be possible to produce invalid in that case, but it is probably too much work for quite a small benefit.