[llvm] [AMDGPU] Fix encoding of VOP3P dpp on GFX11 and GFX12 (PR #82710)

[Jay Foad via llvm-commits]

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

I do not have a deep understanding of VOP3P DPP encodings. But the code change seems obvious, and I spot-checked some of the test diffs against SP3.

https://github.com/llvm/llvm-project/pull/82710