prefer_conditional_assignment false positive on complex statements

[srawlins]

Take the following code:

class Person {
  A a;
  A b;

  void f() {
    if (a.i == null) {
      // LINT
      b.i = 7;
    }
  }
}

class A {
  int i;
}

This code reports an error at the LINT line, "Prefer using ??= over testing for null," because it does not take into account that a.i is not equal to b.i.

(Same cause as #57855 and #57858)