[ADT] Add roundUpNumBuckets to DenseMap (NFC) (#168301)

This patch adds computeNumBuckets, a helper function to compute the
number of buckets.

This is part of the effort outlined in #168255.  This makes it easier
to move the core logic of grow() to DenseMapBase::grow().

Author: kazutakahirata

llvm/include/llvm/ADT/DenseMap.h

@@ -556,7 +556,10 @@ class DenseMapBase : public DebugEpochBase {
     return llvm::make_range(getBuckets(), getBucketsEnd());
   }
 
-  void grow(unsigned AtLeast) { derived().grow(AtLeast); }
+  void grow(unsigned MinNumBuckets) {
+    unsigned NumBuckets = DerivedT::roundUpNumBuckets(MinNumBuckets);
+    derived().grow(NumBuckets);
+  }
 
   template <typename LookupKeyT>
   BucketT *findBucketForInsertion(const LookupKeyT &Lookup,
@@ -840,8 +843,12 @@ class DenseMap : public DenseMapBase<DenseMap<KeyT, ValueT, KeyInfoT, BucketT>,
     NumBuckets = 0;
   }
 
+  static unsigned roundUpNumBuckets(unsigned MinNumBuckets) {
+    return std::max(64u,
+                    static_cast<unsigned>(NextPowerOf2(MinNumBuckets - 1)));
+  }
+
   void grow(unsigned AtLeast) {
-    AtLeast = std::max<unsigned>(64, NextPowerOf2(AtLeast - 1));
     DenseMap Tmp(AtLeast, typename BaseT::ExactBucketCount{});
     Tmp.moveFrom(*this);
     swapImpl(Tmp);
@@ -1106,11 +1113,15 @@ class SmallDenseMap
     new (getLargeRep()) LargeRep{nullptr, 0};
   }
 
-  void grow(unsigned AtLeast) {
-    if (AtLeast > InlineBuckets)
-      AtLeast = std::max<unsigned>(64, NextPowerOf2(AtLeast - 1));
+  static unsigned roundUpNumBuckets(unsigned MinNumBuckets) {
+    if (MinNumBuckets <= InlineBuckets)
+      return MinNumBuckets;
+    return std::max(64u,
+                    static_cast<unsigned>(NextPowerOf2(MinNumBuckets - 1)));
+  }
 
-    SmallDenseMap Tmp(AtLeast, typename BaseT::ExactBucketCount{});
+  void grow(unsigned NumBuckets) {
+    SmallDenseMap Tmp(NumBuckets, typename BaseT::ExactBucketCount{});
     Tmp.moveFrom(*this);
 
     if (Tmp.Small) {