BUG: Series.replace with CoW when made from an Index
- [x] closes #61622 (Replace xxxx with the GitHub issue number)
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When we create a Series from an Index, it's zero copy which means that with CoW there are weak refs to the Index. Comparison of these weak refs uses `Index.__eq__`, which operates on the array (unlike `Block.__eq__` which is merely `is`). This leads to failure in `Series.replace`.

Instead, we replace the equality checks with `is`, plus some additional logic for performance. I believe this is the only place where we are using `__eq__` on these references.
Thanks @rhshadrach 