[llvm] [AMDGPU] Fix negative immediate offset for unbuffered smem loads (PR #89165)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

Looks reasonable to me. I don't fully understand all the logic, but we have tested this in some graphics cases that previously crashed due to abusing negative offsets, and the results were good.

https://github.com/llvm/llvm-project/pull/89165