[ADT] Avoid creating iterators in DenseMap::operator[] (NFC) (llvm#155862)

An attempt to resolve the slowdown from llvm#155204

Author: MacDue

llvm/include/llvm/ADT/DenseMap.h

@@ -348,11 +348,11 @@ class DenseMapBase : public DebugEpochBase {
   }
 
   ValueT &operator[](const KeyT &Key) {
-    return try_emplace_impl(Key).first->second;
+    return lookupOrInsertIntoBucket(Key).first->second;
   }
 
   ValueT &operator[](KeyT &&Key) {
-    return try_emplace_impl(std::move(Key)).first->second;
+    return lookupOrInsertIntoBucket(std::move(Key)).first->second;
   }
 
   /// isPointerIntoBucketsArray - Return true if the specified pointer points
@@ -477,16 +477,24 @@ class DenseMapBase : public DebugEpochBase {
 
 private:
   template <typename KeyArgT, typename... Ts>
-  std::pair<iterator, bool> try_emplace_impl(KeyArgT &&Key, Ts &&...Args) {
+  std::pair<BucketT *, bool> lookupOrInsertIntoBucket(KeyArgT &&Key,
+                                                      Ts &&...Args) {
     BucketT *TheBucket = nullptr;
     if (LookupBucketFor(Key, TheBucket))
-      return {makeInsertIterator(TheBucket), false}; // Already in the map.
+      return {TheBucket, false}; // Already in the map.
 
     // Otherwise, insert the new element.
     TheBucket = findBucketForInsertion(Key, TheBucket);
     TheBucket->getFirst() = std::forward<KeyArgT>(Key);
     ::new (&TheBucket->getSecond()) ValueT(std::forward<Ts>(Args)...);
-    return {makeInsertIterator(TheBucket), true};
+    return {TheBucket, true};
+  }
+
+  template <typename KeyArgT, typename... Ts>
+  std::pair<iterator, bool> try_emplace_impl(KeyArgT &&Key, Ts &&...Args) {
+    auto [Bucket, Inserted] = lookupOrInsertIntoBucket(
+        std::forward<KeyArgT>(Key), std::forward<Ts>(Args)...);
+    return {makeInsertIterator(Bucket), Inserted};
   }
 
   iterator makeIterator(BucketT *P, BucketT *E, DebugEpochBase &Epoch,