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

[Nikita Popov via llvm-commits]

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@@ -845,8 +845,10 @@ class TargetExtType : public Type {
     /// This type may be allocated on the stack, either as the allocated type
     /// of an alloca instruction or as a byval function parameter.
     CanBeLocal = 1U << 2,
-    // This type may be used as an element in a vector.
+    /// This type may be used as an element in a vector.
     CanBeVectorElement = 1U << 3,
+    /// All uses of this type must not attempt to introspect or obscure it.
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nikic wrote:

This is pretty vague... How about something like this?
```suggestion
    /// This type can only be used in intrinsic arguments and return values.
    /// In particular, it cannot be used in select and phi instructions.
```

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