[llvm] [AMDGPU] Fix canonicalization of truncated values. (PR #83054)

[Harald van Dijk via llvm-commits]

hvdijk wrote:

Thanks! If I happen to end up figuring out how to simplify the TableGen I can create a followup PR, otherwise I look forward to seeing how someone else manages to do :)

https://github.com/llvm/llvm-project/pull/83054