[clang] [flang] [llvm] [mlir] [polly] Enforce single-operand form for llvm.loop.vectorize metadata (PR #210932)

[Michael Kruse via cfe-commits]

================
@@ -250,6 +250,26 @@ void llvm::addStringMetadataToLoop(Loop *TheLoop, const char *StringMD,
   TheLoop->setLoopID(NewLoopID);
 }
 
+void llvm::addStringMetadataToLoop(Loop *TheLoop, StringRef StringMD) {
+  LLVMContext &Context = TheLoop->getHeader()->getContext();
+  SmallVector<Metadata *, 4> MDs(1);
+  // Retain existing metadata, skipping a name-only node with the same string.
+  if (MDNode *LoopID = TheLoop->getLoopID())
+    for (const MDOperand &Op : drop_begin(LoopID->operands())) {
+      MDNode *Node = cast<MDNode>(Op);
+      if (Node->getNumOperands() == 1)
+        if (auto *S = dyn_cast<MDString>(Node->getOperand(0)))
+          if (S->getString() == StringMD)
+            return;
+      MDs.push_back(Node);
+    }
+  MDs.push_back(MDNode::get(Context, {MDString::get(Context, StringMD)}));
+  MDNode *NewLoopID = MDNode::get(Context, MDs);
+  // Set operand 0 to refer to the loop id itself.
----------------
Meinersbur wrote:

```suggestion
  // LoopIDs refer to themselves to make each of them unique
```
No description of what that line does is necessary; explain why it is done

https://github.com/llvm/llvm-project/pull/210932