[SQL] Optimize nested `if` expressions with the same condition

The compiler generates code with the following shape:

```rs
if (x) {
   if (x) { a } else { b }
} else {
   c
}
```

This can be simplified to 

```rs
if (x) {
    a
} else {
    c
}
```

This code appears especially in apply nodes, so it is not particularly performance sensitive.