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

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -291,8 +291,7 @@ struct VPTransformState {
 
   /// Set the generated scalar \p V for \p Def and the given \p Lane.
   void set(VPValue *Def, Value *V, const VPLane &Lane) {
-    auto Iter = Data.VPV2Scalars.insert({Def, {}});
-    auto &Scalars = Iter.first->second;
+    auto &Scalars = Data.VPV2Scalars[Def];
     unsigned CacheIdx = Lane.mapToCacheIndex(VF);
     if (Scalars.size() <= CacheIdx)
       Scalars.resize(CacheIdx + 1);