[llvm] [AMDGPU] Treat GEP offsets as signed in AMDGPUPromoteAlloca (PR #157682)

[Fabian Ritter via llvm-commits]

ritter-x2a wrote:

This relates to [this discussion](https://github.com/llvm/llvm-project/pull/155415#issuecomment-3244625707). See the diff from the first commit in the PR to see the previous, wrong behavior in the test.

https://github.com/llvm/llvm-project/pull/157682