[llvm] [HLSL] Allow completely unused cbuffers (PR #164557)

[Helena Kotas via llvm-commits]

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@@ -43,8 +43,13 @@ std::optional<CBufferMetadata> CBufferMetadata::get(Module &M) {
   for (const MDNode *MD : CBufMD->operands()) {
     assert(MD->getNumOperands() && "Invalid cbuffer metadata");
 
-    auto *Handle = cast<GlobalVariable>(
-        cast<ValueAsMetadata>(MD->getOperand(0))->getValue());
+    // For an unused cbuffer, the handle may have been optimizzd out
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hekota wrote:

```suggestion
    // For an unused cbuffer, the handle may have been optimized out
```

https://github.com/llvm/llvm-project/pull/164557