[llvm] Use DenseMap::lookup (NFC)

(cherry picked from commit 3c03a15)

Author: kazutakahirata

llvm/include/llvm/ADT/GenericCycleImpl.h

@@ -370,10 +370,7 @@ void GenericCycleInfo<ContextT>::compute(FunctionT &F) {
 template <typename ContextT>
 auto GenericCycleInfo<ContextT>::getCycle(const BlockT *Block) const
     -> CycleT * {
-  auto MapIt = BlockMap.find(Block);
-  if (MapIt != BlockMap.end())
-    return MapIt->second;
-  return nullptr;
+  return BlockMap.lookup(Block);
 }
 
 /// \brief get the depth for the cycle which containing a given block.

llvm/lib/TextAPI/SymbolSet.cpp

@@ -29,8 +29,5 @@ Symbol *SymbolSet::addGlobal(SymbolKind Kind, StringRef Name, SymbolFlags Flags,
 }
 
 const Symbol *SymbolSet::findSymbol(SymbolKind Kind, StringRef Name) const {
-  auto It = Symbols.find({Kind, Name});
-  if (It != Symbols.end())
-    return It->second;
-  return nullptr;
+  return Symbols.lookup({Kind, Name});
 }

llvm/lib/Transforms/IPO/FunctionSpecialization.cpp

@@ -147,9 +147,7 @@ static Cost estimateBasicBlocks(SmallVectorImpl<BasicBlock *> &WorkList,
 static Constant *findConstantFor(Value *V, ConstMap &KnownConstants) {
   if (auto *C = dyn_cast<Constant>(V))
     return C;
-  if (auto It = KnownConstants.find(V); It != KnownConstants.end())
-    return It->second;
-  return nullptr;
+  return KnownConstants.lookup(V);
 }
 
 Cost InstCostVisitor::getUserBonus(Instruction *User, Value *Use, Constant *C) {