[llvm] [AMDGPU][MC] Implement fft and rotate modes for ds_swizzle_b32 (PR #108064)

[Georg Lehmann via llvm-commits]

DadSchoorse wrote:

I think rotate/fft are unsupported on GFX6/7, so using them should probably result in assembler errors for those generations. (and maybe GFX8, though someone in #28975 said it's supported there, just not mentioned in the documentation)

https://github.com/llvm/llvm-project/pull/108064