[wgsl-in] Function with missing return statement is sometimes accepted without an error

This function is accepted, even though it doesn't return anything. Seems like it should be an error.

```
fn f() -> i32 {
    let x = 1 + 1;
}
```

Strangely, it's only accepted in certain situations. If the `let` is changed to `var` or the expression is simplified to just `1` then it produces a validation error.