[llvm] [DirectX] Reland #142853 with Circular GEP fixes (PR #143747)

[Farzon Lotfi via llvm-commits]

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@@ -271,10 +275,18 @@ bool DXILFlattenArraysVisitor::visitGetElementPtrInstInGEPChainBase(
         genInstructionFlattenIndices(GEPInfo.Indices, GEPInfo.Dims, Builder);
 
   ArrayType *FlattenedArrayType = GEPInfo.ParentArrayType;
+  
+  // Don't append '.flat' to an empty string. If the SSA name isn't available
+  // it could conflict with the ParentOperand's name.
+  std::string FlatName = GEP.hasName() ? GEP.getName().str() + ".flat" : "";
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farzonl wrote:

did `.str()` in this case because clangd warned that `getName()` alone could cause a potential use after free.

https://github.com/llvm/llvm-project/pull/143747