Conditional assignment merging can move a `this` member target ahead of a `super()` condition. In a derived constructor, that rewrites valid source into an access of uninitialized `this`, causing a `ReferenceError`.

Keep reorder-sensitive `this` targets guarded until the first unconditional top-level `super()` call in a derived constructor. This preserves the optimization after `super()` has initialized `this`, including the real-world Terser fixture that caused the earlier size increase.

## Example

Input:

```js
class B extends A {
  constructor() {
    super() ? (this.b = 0) : (this.b = 1);
  }
}
```

Before:

```js
class B extends A{constructor(){this.b=+!super()}}
```

After:

```js
class B extends A{constructor(){super()?this.b=0:this.b=1}}
```

Verified with the full `oxc_minifier` suite (625 passed, 42 ignored), Clippy with warnings denied, formatting, `just minsize`, allocation regeneration, and byte-for-byte comparison of all 12 minsize outputs against `main`. Minsize and allocation snapshots are unchanged.

Addresses the wrong-code issue raised in #24706 (comment).

Implemented with AI assistance (OpenAI Codex). The contributor remains responsible for reviewing, understanding, and submitting the changes under the repository AI usage policy.