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

[Nikita Popov via llvm-commits]

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nikic wrote:

I'd like to have a SimplifyCFG test that shows that no incorrect replacement occurs. I *think* this test is basically that? But this is a DirectX backend test. We should test this with `-passes=simplifycfg` in llvm/test/Transforms/SimplifyCFG.

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