PERF: Faster comparisons of indexes when compared to self
Examples:

```python
>>> rng = pd.RangeIndex(100_000)
>>> %timeit rng == rng
429 µs ± 96.9 µs per loop  # master
14.1 µs ± 93.8 ns per loop  # this PR
>>> idx = rng.astype(int)
>>> %timeit idx == idx
347 µs ± 1.42 µs per loop  #master
14.1 µs ± 91.4 ns per loop  # this PR
>>> idx = rng.astype(object)
>>> %timeit idx == idx
5.97 ms ± 12.7 µs per loop  # master
3.72 ms ± 28.4 µs per loop  # this PR
```
