[VPlan] Assert that addMetadata does not add duplicates (NFC).

Duplicates are not supported, and would require kind-specific merging of
metadata.

Author: fhahn

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -959,6 +959,11 @@ class VPIRMetadata {
 
   /// Add metadata with kind \p Kind and \p Node.
   void addMetadata(unsigned Kind, MDNode *Node) {
+    assert(none_of(Metadata,
+                   [Kind](const std::pair<unsigned, MDNode *> &P) {
+                     return P.first == Kind;
+                   }) &&
+           "Kind must appear at most once in Metadata");
     Metadata.emplace_back(Kind, Node);
   }