Fix regression for setitem not aligning rhs with boolean indexer
- [x] closes #39931
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

reindexing solves the issue here before dispatching to iloc
> reindexing solves the issue here before dispatching to iloc

maybe add words to that effect as a comment in the code

test failure unreleated. restarted job Added something thanks @phofl  @meeseeksdev backport 1.2.x