[mlir] Avoid repeated hash lookups (NFC) (#107518)

Author: kazutakahirata

mlir/include/mlir/Dialect/Transform/IR/TransformDialect.h

@@ -298,13 +298,10 @@ void TransformDialect::addTypeIfNotRegistered() {
 template <typename DataTy>
 DataTy &TransformDialect::getOrCreateExtraData() {
   TypeID typeID = TypeID::get<DataTy>();
-  auto it = extraData.find(typeID);
-  if (it != extraData.end())
-    return static_cast<DataTy &>(*it->getSecond());
-
-  auto emplaced =
-      extraData.try_emplace(typeID, std::make_unique<DataTy>(getContext()));
-  return static_cast<DataTy &>(*emplaced.first->getSecond());
+  auto [it, inserted] = extraData.try_emplace(typeID);
+  if (inserted)
+    it->getSecond() = std::make_unique<DataTy>(getContext());
+  return static_cast<DataTy &>(*it->getSecond());
 }
 
 /// A wrapper for transform dialect extensions that forces them to be