[llvm] [AMDGPU Use BUF multiclasses to reduce repetition. NFC. (PR #84003)

[Ivan Kosarev via llvm-commits]

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

Looks great to me! Also eliminates the need to guess whether it's `def` or `defm` when referring to `_Real` (mutli)classes. (Nit: the title misses `]`.)

https://github.com/llvm/llvm-project/pull/84003