[llvm] [AMDGPU] Remove s_set_inst_prefetch_distance support from GFX12 (PR #78786)

[Jay Foad via llvm-commits]

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

Please update (or remove) the comment in `GCNSubtarget::hasInstPrefetch`.

https://github.com/llvm/llvm-project/pull/78786