[ValueMapper] NFC: Remove dead code to pause metadata mapping

[pirama]

Summary

This functionality was added when Mapper::mapMetadata was recursive. It
is no longer needed after r265456, which switched it to be iterative.

[dexonsmith]

LGTM, thanks!

llvm/include/llvm/IR/ValueMap.h

class ValueMap {
   using ValueMapCVH = ValueMapCallbackVH<KeyT, ValueT, Config>;
   using MapT = DenseMap<ValueMapCVH, ValueT, DenseMapInfo<ValueMapCVH>>;
   using MDMapT = DenseMap<const Metadata *, TrackingMDRef>;
   using ExtraData = typename Config::ExtraData;
 
   MapT Map;
   Optional<MDMapT> MDMap;
   ExtraData Data;
-  bool MayMapMetadata = true;
 
 public:
   using key_type = KeyT;
   using mapped_type = ValueT;
   using value_type = std::pair<KeyT, ValueT>;
   using size_type = unsigned;
 
   explicit ValueMap(unsigned NumInitBuckets = 64)
public:
   bool hasMD() const { return bool(MDMap); }
   MDMapT &MD() {
     if (!MDMap)
       MDMap.emplace();
     return *MDMap;
   }
   Optional<MDMapT> &getMDMap() { return MDMap; }
 
-  bool mayMapMetadata() const { return MayMapMetadata; }
-  void enableMapMetadata() { MayMapMetadata = true; }
-  void disableMapMetadata() { MayMapMetadata = false; }
-
   /// Get the mapped metadata, if it's in the map.
   Optional<Metadata *> getMappedMD(const Metadata *MD) const {
     if (!MDMap)
       return None;
     auto Where = MDMap->find(MD);
     if (Where == MDMap->end())
       return None;
     return Where->second.get();

llvm/lib/Transforms/Utils/ValueMapper.cpp

Metadata *MDNodeMapper::mapTopLevelUniquedNode(const MDNode &FirstN) {
 
   // Map all the nodes in the graph.
   mapNodesInPOT(G);
 
   // Return the original node, remapped.
   return *getMappedOp(&FirstN);
 }
 
-namespace {
-
-struct MapMetadataDisabler {
-  ValueToValueMapTy &VM;
-
-  MapMetadataDisabler(ValueToValueMapTy &VM) : VM(VM) {
-    VM.disableMapMetadata();
-  }
-
-  ~MapMetadataDisabler() { VM.enableMapMetadata(); }
-};
-
-} // end anonymous namespace
-
 Optional<Metadata *> Mapper::mapSimpleMetadata(const Metadata *MD) {
   // If the value already exists in the map, use it.
   if (Optional<Metadata *> NewMD = getVM().getMappedMD(MD))
     return *NewMD;
 
   if (isa<MDString>(MD))
     return const_cast<Metadata *>(MD);
 
   // This is a module-level metadata.  If nothing at the module level is
   // changing, use an identity mapping.
   if ((Flags & RF_NoModuleLevelChanges))
     return const_cast<Metadata *>(MD);
 
   if (auto *CMD = dyn_cast<ConstantAsMetadata>(MD)) {
-    // Disallow recursion into metadata mapping through mapValue.
-    MapMetadataDisabler MMD(getVM());
-
     // Don't memoize ConstantAsMetadata.  Instead of lasting until the
     // LLVMContext is destroyed, they can be deleted when the GlobalValue they
     // reference is destructed.  These aren't super common, so the extra
     // indirection isn't that expensive.
     return wrapConstantAsMetadata(*CMD, mapValue(CMD->getValue()));
   }
 
   assert(isa<MDNode>(MD) && "Expected a metadata node");