[llvm] [AVR] Fix incorrect selection of post indexed addresses (PR #145040)

[Ben Shi via llvm-commits]

benshi001 wrote:

> > This PR could just be the new CI test?
> 
> Yea, if you dropped the three commits here and added a new commit just with the test, I could take it from here and create a new pull request with the test and fix 🙂

We need not two commits (test+fix) for a bug fix, you can directly create a new PR with both of the test and fix. 

https://github.com/llvm/llvm-project/pull/145040