bvi_tailrec not firing when it should

Consider a function like
```
fun fib n =
case n of
0 => 0
| 1 => 1
| n => fib (n - 1) + fib (n - 2);
```
It should get optimized to
```
fun fib_acc a n =
case n of
0 => 0
| 1 => 1
| n => fib_acc (a + fib (n - 2)) (n - 1))
fun fib n = fib_acc 0 n
```