Stack overflow on option of a recursive @unboxed variant #9018

The compiler crashes with a stack overflow when it decides whether option of a recursive @unboxed variant can be unboxed.

@unboxed type rec t = Recur(t) | End
let x: option<t> = Some(End)

Fatal error: exception Stack overflow

Observed on master (d53b8ae).

Typeopt.type_cannot_contain_undefined checks the payload of every unboxed case, and Recur's payload is t again. The walk has no visited set or fuel, so it never ends.

Expected: the program compiles. t's values are "End" or, recursively, values of t, so none of them is undefined, and Some(End) can be unboxed.

Found by Codex on #9017, where @notUndefined type alias = t reaches the same walk.