Bug in iloc aligned objects
- [x] closes #22046
- [x] closes #37593
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

If iloc should be purely position based, this PR fixes the bug described in the issue. One test depends on the wrong behavior, so I had to adjust it.