[llvm] Use *Map::try_emplace (NFC) (llvm#143321)

- try_emplace(Key) is shorter than insert(std::make_pair(Key, 0)).
- try_emplace performs value initialization without value parameters.
- We overwrite values on successful insertion anyway.

Author: kazutakahirata

llvm/lib/Transforms/ObjCARC/BlotMapVector.h

@@ -53,8 +53,7 @@ template <class KeyT, class ValueT> class BlotMapVector {
   const_iterator end() const { return Vector.end(); }
 
   ValueT &operator[](const KeyT &Arg) {
-    std::pair<typename MapTy::iterator, bool> Pair =
-        Map.insert(std::make_pair(Arg, size_t(0)));
+    std::pair<typename MapTy::iterator, bool> Pair = Map.try_emplace(Arg);
     if (Pair.second) {
       size_t Num = Vector.size();
       Pair.first->second = Num;

llvm/lib/Transforms/Scalar/ConstantHoisting.cpp

@@ -381,7 +381,7 @@ void ConstantHoistingPass::collectConstantCandidates(
     ConstCandMapType::iterator Itr;
     bool Inserted;
     ConstPtrUnionType Cand = ConstInt;
-    std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0));
+    std::tie(Itr, Inserted) = ConstCandMap.try_emplace(Cand);
     if (Inserted) {
       ConstIntCandVec.push_back(ConstantCandidate(ConstInt));
       Itr->second = ConstIntCandVec.size() - 1;
@@ -439,7 +439,7 @@ void ConstantHoistingPass::collectConstantCandidates(
   ConstCandMapType::iterator Itr;
   bool Inserted;
   ConstPtrUnionType Cand = ConstExpr;
-  std::tie(Itr, Inserted) = ConstCandMap.insert(std::make_pair(Cand, 0));
+  std::tie(Itr, Inserted) = ConstCandMap.try_emplace(Cand);
   if (Inserted) {
     ExprCandVec.push_back(ConstantCandidate(
         ConstantInt::get(Type::getInt32Ty(*Ctx), Offset.getLimitedValue()),

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -2788,7 +2788,7 @@ std::pair<size_t, Immediate> LSRInstance::getUse(const SCEV *&Expr,
   }
 
   std::pair<UseMapTy::iterator, bool> P =
-    UseMap.insert(std::make_pair(LSRUse::SCEVUseKindPair(Expr, Kind), 0));
+      UseMap.try_emplace(LSRUse::SCEVUseKindPair(Expr, Kind));
   if (!P.second) {
     // A use already existed with this base.
     size_t LUIdx = P.first->second;