Recursive sequence expression containing recursive definition doesn't run in finite space

This small example causes a stack overflow, but shouldn't:

```
let rec loop r = seq {
if r > 0 then
let rec unused() = unused()
yield r
yield! loop r
}

printfn "%i" (Seq.nth 10000000 (loop 1))
```

Removing the conditional or replacing the unused recursive definition with an unused non-recursive definition both appear to result in the expected behavior of constant stack space usage.