[YAML][NFC] Replace if-else with switch in createHNodes

[Amir]

Summary

BOLT YAML profile reading time gets marginally faster (14.1572->13.9207 s) for
a large YAML profile (121MB/31K functions). Not claiming stat significance
though.

[Amir]

@njames93, @silvas, @grimar, @thomasfinch, @Bigcheese, @hintonda – folks, I checked blame on this function and picked you as most recent authors and reviewers.

I'm also looking for suggestions on how to speed up this function and parsing YAML in general. First thing that stands out in the profile is that we do allocate a lot of Nodes. But since they're allocated using BumpPtrAllocator I don't see much room for improvement here:
https://github.com/llvm/llvm-project/blob/49dfbc6efc64bcd9031bc80fd20f4be080ebf56e/llvm/include/llvm/Support/YAMLParser.h#L141

[hintonda]

LGTM.

llvm/lib/Support/YAMLTraits.cpp

 }
 
 void Input::reportWarning(const SMRange &range, const Twine &message) {
   Strm->printError(range, message, SourceMgr::DK_Warning);
 }
 
 std::unique_ptr<Input::HNode> Input::createHNodes(Node *N) {
   SmallString<128> StringStorage;
-  if (ScalarNode *SN = dyn_cast<ScalarNode>(N)) {
+  switch (N->getType()) {
+  case Node::NK_Scalar: {
+    ScalarNode *SN = dyn_cast<ScalarNode>(N);
     StringRef KeyStr = SN->getValue(StringStorage);
     if (!StringStorage.empty()) {
       // Copy string to permanent storage
       KeyStr = StringStorage.str().copy(StringAllocator);
     }
     return std::make_unique<ScalarHNode>(N, KeyStr);
-  } else if (BlockScalarNode *BSN = dyn_cast<BlockScalarNode>(N)) {
+  }
+  case Node::NK_BlockScalar: {
+    BlockScalarNode *BSN = dyn_cast<BlockScalarNode>(N);
     StringRef ValueCopy = BSN->getValue().copy(StringAllocator);
     return std::make_unique<ScalarHNode>(N, ValueCopy);
-  } else if (SequenceNode *SQ = dyn_cast<SequenceNode>(N)) {
+  }
+  case Node::NK_Sequence: {
+    SequenceNode *SQ = dyn_cast<SequenceNode>(N);
     auto SQHNode = std::make_unique<SequenceHNode>(N);
     for (Node &SN : *SQ) {
       auto Entry = createHNodes(&SN);
       if (EC)
         break;
       SQHNode->Entries.push_back(std::move(Entry));
     }
     return std::move(SQHNode);
-  } else if (MappingNode *Map = dyn_cast<MappingNode>(N)) {
+  }
+  case Node::NK_Mapping: {
+    MappingNode *Map = dyn_cast<MappingNode>(N);
     auto mapHNode = std::make_unique<MapHNode>(N);
     for (KeyValueNode &KVN : *Map) {
       Node *KeyNode = KVN.getKey();
       ScalarNode *Key = dyn_cast_or_null<ScalarNode>(KeyNode);
       Node *Value = KVN.getValue();
       if (!Key || !Value) {
         if (!Key)
           setError(KeyNode, "Map key must be a scalar");
for (KeyValueNode &KVN : *Map) {
         setError(KeyNode, Twine("duplicated mapping key '") + KeyStr + "'");
       auto ValueHNode = createHNodes(Value);
       if (EC)
         break;
       mapHNode->Mapping[KeyStr] =
           std::make_pair(std::move(ValueHNode), KeyNode->getSourceRange());
     }
     return std::move(mapHNode);
-  } else if (isa<NullNode>(N)) {
+  }
+  case Node::NK_Null:
     return std::make_unique<EmptyHNode>(N);
-  } else {
+  default:
     setError(N, "unknown node kind");
     return nullptr;
   }
 }
 
 void Input::setError(const Twine &Message) {
   setError(CurrentNode, Message);
 }