[HLSL] Allow completely unused cbuffers

We were checking for cbuffers where the global was removed, but if the
buffer is completely unused the whole thing can be null.

Author: bogner

llvm/lib/Frontend/HLSL/CBuffer.cpp

@@ -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
+    Metadata *OpMD = MD->getOperand(0);
+    if (!OpMD)
+      continue;
+
+    auto *Handle =
+        cast<GlobalVariable>(cast<ValueAsMetadata>(OpMD)->getValue());
     CBufferMapping &Mapping = Result->Mappings.emplace_back(Handle);
 
     for (int I = 1, E = MD->getNumOperands(); I < E; ++I) {

llvm/test/CodeGen/DirectX/CBufferAccess/unused.ll

@@ -0,0 +1,13 @@
+; RUN: opt -S -dxil-cbuffer-access -mtriple=dxil--shadermodel6.3-library %s | FileCheck %s
+; Check that we correctly ignore cbuffers that were nulled out by optimizations.
+
+%__cblayout_CB = type <{ float }>
+@CB.cb = local_unnamed_addr global target("dx.CBuffer", %__cblayout_CB) poison
+@x = external local_unnamed_addr addrspace(2) global float, align 4
+
+; CHECK-NOT: !hlsl.cbs =
+!hlsl.cbs = !{!0, !1, !2}
+
+!0 = !{ptr @CB.cb, ptr addrspace(2) @x}
+!1 = !{ptr @CB.cb, null}
+!2 = !{null, null}