[ADT] Add DenseMap::deallocateBuckets (NFC)

This patch adds a small helper function DenseMap::deallocateBuckets
just like SmallDenseMap::deallocateBuckets.

With the new helper function:

  ~DenseMap()
  DenseMap &operator=(DenseMap &&other)

will look identical to their respective SmallDenseMap counterparts,
facilitating further refactoring.

Author: kazutakahirata

llvm/include/llvm/ADT/DenseMap.h

@@ -738,7 +738,7 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
 
   ~DenseMap() {
     this->destroyAll();
-    deallocate_buffer(Buckets, sizeof(BucketT) * NumBuckets, alignof(BucketT));
+    deallocateBuckets();
   }
 
   void swap(DenseMap &RHS) {
@@ -758,15 +758,15 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
 
   DenseMap &operator=(DenseMap &&other) {
     this->destroyAll();
-    deallocate_buffer(Buckets, sizeof(BucketT) * NumBuckets, alignof(BucketT));
+    deallocateBuckets();
     init(0);
     swap(other);
     return *this;
   }
 
   void copyFrom(const DenseMap &other) {
     this->destroyAll();
-    deallocate_buffer(Buckets, sizeof(BucketT) * NumBuckets, alignof(BucketT));
+    deallocateBuckets();
     if (allocateBuckets(other.NumBuckets)) {
       this->BaseT::copyFrom(other);
     } else {
@@ -827,6 +827,10 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
 
   unsigned getNumBuckets() const { return NumBuckets; }
 
+  void deallocateBuckets() {
+    deallocate_buffer(Buckets, sizeof(BucketT) * NumBuckets, alignof(BucketT));
+  }
+
   bool allocateBuckets(unsigned Num) {
     NumBuckets = Num;
     if (NumBuckets == 0) {