`-Wenum-enum-conversion` missing for compound assignments

```
enum E1 { A };
enum E2 { B };

void test(enum E1 e1, enum E2 e2) {
  e1 = e1 | e2; // warning: bitwise operation between different enumeration types
  e1 |= e2; // no warning 
}
```

clang should emit warning `-Wenum-enum-conversion` for the compound assignment too since it also performs a bitwise operation between `E1` and `E2`.