Using hole var `_` leads to most other vars in tensor assignment being marked as unused in FunC

Example:
```func
int main() {
    return 0;
}

(int, int) foo() {
    return (1, 2);
}

(int, int) get_foo() {
    (
        _,
        int b
    ) = foo();

    return 1 + b;
}
```

`b` will be marked as unused.

However note that in some cases some vars are marked used correctly.