[ADT] Refactor DenseMapIterator::operator-> (NFC) (#157227)

This patchs implement DenseMapIterator::operator-> in terms of
operator* to reduce code duplication.

Author: kazutakahirata

llvm/include/llvm/ADT/DenseMap.h

@@ -1242,13 +1242,7 @@ class DenseMapIterator : DebugEpochBase::HandleBase {
       return Ptr[-1];
     return *Ptr;
   }
-  pointer operator->() const {
-    assert(isHandleInSync() && "invalid iterator access!");
-    assert(Ptr != End && "dereferencing end() iterator");
-    if (shouldReverseIterate<KeyT>())
-      return &(Ptr[-1]);
-    return Ptr;
-  }
+  pointer operator->() const { return &operator*(); }
 
   friend bool operator==(const DenseMapIterator &LHS,
                          const DenseMapIterator &RHS) {