- Add assertions that the iterators are either (a) from the same container or (b) both default constructed. Standard says: "The domain of == for forward iterators is that of iterators over the same underlying sequence. However, value-initialized iterators may be compared and shall compare equal to other value-initialized iterators of the same type." - [reference](https://eel.is/c++draft/forward.iterators#2).
- Also optimize IsEndIterator a bit.

PiperOrigin-RevId: 487617518
Change-Id: Iefba5c3bc8caa93954671793e6929e22f419c298