[llvm] [DirectX] Make dx.RawBuffer an op that can't be replaced (PR #154620)

[Justin Bogner via llvm-commits]

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

This looks good to me. Please wait to hear back from @nikic on whether we should do the `isTokenLikeTy()` refactoring before or after this change.

https://github.com/llvm/llvm-project/pull/154620