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

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

> 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.

I'd like the usages in the IR verifier to be adjusted, and an IR verifier test added that errors when using such a type in a phi node.

I don't think we need to replace other uses at this time.

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