loose fvar in `split`

```lean
example (b : Bool) :
    (if b then 1 else if b then 1 else 0) = (if b then 1 else 0) := by
  split <;> sorry
```
```
(kernel) declaration has free variables '_example'
```

Observed on `leanprover/lean4:v4.5.0-rc1`