[llvm] [HLSL] Adding support for root descriptors in root signature metadata representation (PR #139781)

[Justin Bogner via llvm-commits]

================
@@ -55,6 +55,14 @@ static std::optional<uint32_t> extractMdIntValue(MDNode *Node,
   return std::nullopt;
 }
 
+static std::optional<StringRef> extractMdStringValue(MDNode *Node,
+                                                     unsigned int OpId) {
+  MDString *NodeText = cast<MDString>(Node->getOperand(OpId));
+  if (NodeText == nullptr)
+    return std::nullopt;
----------------
bogner wrote:

This doesn't look correct. `cast<>` will assert if the type is wrong, not return null, so this condition is unreachable (except maybe if `Node` itself is null?). I think you meant to use `dyn_cast` here.

I suspect this is why a test case like the following currently crashes:
```llvm
!dx.rootsignatures = !{!0}
!0 = !{ ptr @main, !1 }
!1 = !{ !2 }
!2 = !{ i32 0 }
```

https://github.com/llvm/llvm-project/pull/139781