REGR: __setitem__ with integer slices on Int/RangeIndex is broken (label instead of positional)
There's an backward incompatible change in pandas 1.0 that I didn't find in the changelog. I might have just overlooked it though.

```python
import numpy as np
X = pd.DataFrame(np.zeros((100, 1)))
X[-4:] = 1
X
```
In pandas 0.25.3 or lower, this results in the last four entries of X to be 1 and all the others zero. In pandas 1.0, it results in all entries of X being 1.
I assume it's a change of indexing axis 0 or axis 1?