[Analysis] Simplify code with DenseMap::operator[] (NFC)

llvm/lib/Analysis/BasicAliasAnalysis.cpp

@@ -219,10 +219,8 @@ bool EarliestEscapeInfo::isNotCapturedBefore(const Value *Object,
     Instruction *EarliestCapture = FindEarliestCapture(
         Object, *const_cast<Function *>(DT.getRoot()->getParent()),
         /*ReturnCaptures=*/false, /*StoreCaptures=*/true, DT);
-    if (EarliestCapture) {
-      auto Ins = Inst2Obj.insert({EarliestCapture, {}});
-      Ins.first->second.push_back(Object);
-    }
+    if (EarliestCapture)
+      Inst2Obj[EarliestCapture].push_back(Object);
     Iter.first->second = EarliestCapture;
   }
 

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -512,10 +512,8 @@ void RuntimePointerChecking::groupChecks(
   unsigned TotalComparisons = 0;
 
   DenseMap<Value *, SmallVector<unsigned>> PositionMap;
-  for (unsigned Index = 0; Index < Pointers.size(); ++Index) {
-    auto [It, _] = PositionMap.insert({Pointers[Index].PointerValue, {}});
-    It->second.push_back(Index);
-  }
+  for (unsigned Index = 0; Index < Pointers.size(); ++Index)
+    PositionMap[Pointers[Index].PointerValue].push_back(Index);
 
   // We need to keep track of what pointers we've already seen so we
   // don't process them twice.

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -1564,8 +1564,7 @@ static void insertFoldCacheEntry(
     UserIDs.pop_back();
     I.first->second = S;
   }
-  auto R = FoldCacheUser.insert({S, {}});
-  R.first->second.push_back(ID);
+  FoldCacheUser[S].push_back(ID);
 }
 
 const SCEV *