[red-knot] Incorrect control flow modelling in if-elif chains

We currently infer a type of `Literal[1, 2, 4]` in the following example, but `1` is not a possible value of `x`. And `3` *is* a possible value for `x`.

```py
def check(x: int) -> bool: ...

if check(x := 1):
    x = 2
elif check(x := 3):
    x = 4

reveal_type(x)  # revealed: Literal[1, 2, 4]; should be Literal[2, 3, 4]
```