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

[Farzon Lotfi via llvm-commits]

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farzonl wrote:

The DX Buffer types are the only `TargetExtType` that uses the `IsTokenLike` property. And the DX target is still an experimental target. I could put a test in `llvm/test/Transforms/SimplifyCFG/` but it would need a requires for the DirectX target. 

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