[llvm] [AMDGPU] Change default loop alignment (PR #155343)

[via llvm-commits]

hjagasiaAMD wrote:

[triton-aiter.xlsx](https://github.com/user-attachments/files/21994782/triton-aiter.xlsx)
Triton Aiter results, especially for unbatched Llama, see a substantial performance hit

I see three options
- Warn the user when they have sub-optimal alignment and give a flag to user (though no easy for user to know whether the flag will help upfront - as alignment can have side effects and can be different impact on different architectures. Saw some cases where nops in unexecuted code paths changed alignment of subsequent executed branch targets hurting performance) (this would also mean we would introduce yet another backend flag)
- Abandon the PR

Would like reviewer suggestions

https://github.com/llvm/llvm-project/pull/155343