[compiler] Fix for scopes with unreachable fallthroughs

Fixes #34108. If a scope ends with with a conditional where some/all branches exit via labeled break, we currently compile in a way that works but bypasses memoization. We end up with a shape like


```js
let t0;
label: {
 if (changed) {
   ...
   if (cond) {
     t0 = ...;
     break label;
   }
   // we don't save the output if the break happens!
   t0 = ...;
   $[0] = t0;
 } else {
   t0 = $[0];
}
```

The fix here is to update AlignReactiveScopesToBlockScopes to take account of breaks that don't go to the natural fallthrough. In this case, we take any active scopes and extend them to start at least as early as the label, and extend at least to the label fallthrough. Thus we produce the correct:

```js
let t0;
if (changed) {
  label: {
    ...
    if (cond) {
      t0 = ...;
      break label;
    }
    t0 = ...;
  }
  // now the break jumps here, and we cache the value
  $[0] = t0;
} else {
  t0 = $[0];
}
```

Note that this messes with one situation: conditional return inside a useMemo. That desugars to a labeled break, and the fix here causes the scope to extend unnecessarily. So we likely want to be a bit more targeted than what i'm doing here. WIP.