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

[Farzon Lotfi via llvm-commits]

================
@@ -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.
----------------
farzonl wrote:

Does that mean the docs for tokenType should be updated
```
The token type is used when a value is associated with an 
instruction but all uses of the value must not attempt to introspect or obscure it. 
As such, it is not appropriate to have a [phi](https://llvm.org/docs/LangRef.html#i-phi) 
or [select](https://llvm.org/docs/LangRef.html#i-select) of type token.
```

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