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

[Jun Wang via llvm-commits]

jwanggit86 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)

FFT/ROTATE are now disallowed for pre-gfx9 targets.

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