[mlir] Fix merging of delayed registrations during DialectRegistry::appendTo

[springerm]

Summary

The existing implementation called DenseMap::insert, which is a no-op if the map already contains an entry with the same key.

[mehdi_amini]

Nice find!

mlir/include/mlir/IR/Dialect.h

public:
 
   // Register all dialects available in the current registry with the registry
   // in the provided context.
   void appendTo(DialectRegistry &destination) const {
     for (const auto &nameAndRegistrationIt : registry)
       destination.insert(nameAndRegistrationIt.second.first,
                          nameAndRegistrationIt.first,
                          nameAndRegistrationIt.second.second);
-    destination.interfaces.insert(interfaces.begin(), interfaces.end());
+    // Merge interfaces.
+    for (auto it : interfaces) {
+      TypeID dialect = it.first;
+      auto destInterfaces = destination.interfaces.find(dialect);
+      if (destInterfaces == destination.interfaces.end()) {
+        destination.interfaces[dialect] = it.second;
+        continue;
+      }
+      // The destination already has delayed interface registrations for this
+      // dialect. Merge registrations into the destination registry.
+      destInterfaces->second.dialectInterfaces.append(
+          it.second.dialectInterfaces.begin(),
+          it.second.dialectInterfaces.end());
+      destInterfaces->second.objectInterfaces.append(
+          it.second.objectInterfaces.begin(), it.second.objectInterfaces.end());
+    }
   }
 
   /// Return the names of dialects known to this registry.
   auto getDialectNames() const {
     return llvm::map_range(
         registry,
         [](const MapTy::value_type &item) -> StringRef { return item.first; });
   }