[Analysis] Avoid repeated hash lookups (NFC) (#126011)

Author: kazutakahirata

llvm/include/llvm/Analysis/BlockFrequencyInfoImpl.h

@@ -1143,14 +1143,15 @@ void BlockFrequencyInfoImpl<BT>::calculate(const FunctionT &F,
 template <class BT>
 void BlockFrequencyInfoImpl<BT>::setBlockFreq(const BlockT *BB,
                                               BlockFrequency Freq) {
-  if (Nodes.count(BB))
-    BlockFrequencyInfoImplBase::setBlockFreq(getNode(BB), Freq);
+  auto [It, Inserted] = Nodes.try_emplace(BB);
+  if (!Inserted)
+    BlockFrequencyInfoImplBase::setBlockFreq(It->second.first, Freq);
   else {
     // If BB is a newly added block after BFI is done, we need to create a new
     // BlockNode for it assigned with a new index. The index can be determined
     // by the size of Freqs.
     BlockNode NewNode(Freqs.size());
-    Nodes[BB] = {NewNode, BFICallbackVH(BB, this)};
+    It->second = {NewNode, BFICallbackVH(BB, this)};
     Freqs.emplace_back();
     BlockFrequencyInfoImplBase::setBlockFreq(NewNode, Freq);
   }