[llvm] AMDGPU: Flat instructions do not have signed offsets (PR #95852)

[Jay Foad via llvm-commits]

https://github.com/jayfoad approved this pull request.

> Flat instructions do not have signed offsets

They do in GFX12. "Signed" was a terrible way to name these patterns. They should be named after the instruction variants that they apply to.

Anyway patch LGTM.

https://github.com/llvm/llvm-project/pull/95852