[llvm] [AVR] Adapt getPostIndexedAddressParts() and getPreIndexedAddressParts (PR #145040)

[Ben Shi via llvm-commits]

benshi001 wrote:

To be honest, I do not prefer this solution, since it is so tricky. Though I used to commit walkaround solutions to llvm-AVR, all of them were clear and easy to understand.

For this solution, it neither fixes the root cause nor explains why it work.

For you tests program, the generated `.ll` files have slight difference,
![image](https://github.com/user-attachments/assets/1dfcc195-144d-4b5a-9bbc-7e005cd0f49f)

Even we can not fix the root cause by now, our walk around solution should at least reveal why `ptr inttoptr (i16 1 to ptr)` is lowerred to wrong assembly.

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