[Vectorize] Simplify code with DenseMap::operator[] (NFC) (#115635)

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlanUnroll.cpp

@@ -297,12 +297,12 @@ void UnrollState::unrollRecipeByUF(VPRecipeBase &R) {
     if (auto *Red = dyn_cast<VPReductionRecipe>(&R)) {
       auto *Phi = cast<VPReductionPHIRecipe>(R.getOperand(0));
       if (Phi->isOrdered()) {
-        auto Ins = VPV2Parts.insert({Phi, {}});
+        auto &Parts = VPV2Parts[Phi];
         if (Part == 1) {
-          Ins.first->second.clear();
-          Ins.first->second.push_back(Red);
+          Parts.clear();
+          Parts.push_back(Red);
         }
-        Ins.first->second.push_back(Copy->getVPSingleValue());
+        Parts.push_back(Copy->getVPSingleValue());
         Phi->setOperand(1, Copy->getVPSingleValue());
       }
     }