"Used before initialization" error false positives and negatives with optional properties

@johnfav03 is already working on this.

Since Aug 6, 2025.

### 🔎 Search Terms

used before its initialization

### 🕗 Version & Regression Information

- This changed between versions 4.2.3 and 4.3.5
- This changed in commit or PR #43100 ([`0967f3b`](https://github.com/microsoft/TypeScript/pull/43100/commits/0967f3bac1d82b61d93090eed8e81aa401426d82))

### ⏯ Playground Link

https://www.typescriptlang.org/play/?useDefineForClassFields=true&ts=5.8.3#code/MYGwhgzhAEDC0G8BQ1XTNAvNALgCwEsIA6AIwG4U1SB+ALmgDsBXAW1IFMAnLaARgBMAZkoBfJElCQYAEURVUYBi3bdoAH2jNGAEw4AzAow47KaaKV74ixDNkEik4oA

### 💻 Code

```ts
class C {
a = this.b;
b?: number = 123;
}

class D {
a: number | undefined;
b = this.a = 123;
}
```

### 🙁 Actual behavior

When initializing `C.a`: no error.

When initializing `D.b`:

```
Property 'a' is used before its initialization. (2729)
```

### 🙂 Expected behavior

When initializing `C.a`:

```
Property 'b' is used before its initialization. (2729)
```

When initializing `D.b`: no error.

### Additional information about the issue

Tried with both `useDefineForClassFields` set to `true` and `false`.

The behavior is as expected when switching the use of `undefined` and the "optional" `?` modifier like this:

```ts
class C {
a = this.b;
b: number | undefined = 123;
}

class D {
a?: number;
b = this.a = 123;
}
```

#43100 seems to have taken a wrong approach. Instead of basing the decision to report an error on whether the `?` modifier is present, it should be based on whether the property access is on the left-hand side of an assignment expression.