[SLP][NFC]Build value-to-gather-nodes map during nodes building, NFC.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -2883,6 +2883,10 @@ class BoUpSLP {
       assert(!BundleMember && "Bundle and VL out of sync");
     } else {
       MustGather.insert(VL.begin(), VL.end());
+      // Build a map for gathered scalars to the nodes where they are used.
+      for (Value *V : VL)
+        if (!isConstant(V))
+          ValueToGatherNodes.try_emplace(V).first->getSecond().insert(Last);
     }
 
     if (UserTreeIdx.UserTE)
@@ -8865,16 +8869,6 @@ static T *performExtractsShuffleAction(
 }
 
 InstructionCost BoUpSLP::getTreeCost(ArrayRef<Value *> VectorizedVals) {
-  // Build a map for gathered scalars to the nodes where they are used.
-  ValueToGatherNodes.clear();
-  for (const std::unique_ptr<TreeEntry> &EntryPtr : VectorizableTree) {
-    if (EntryPtr->State != TreeEntry::NeedToGather)
-      continue;
-    for (Value *V : EntryPtr->Scalars)
-      if (!isConstant(V))
-        ValueToGatherNodes.try_emplace(V).first->getSecond().insert(
-            EntryPtr.get());
-  }
   InstructionCost Cost = 0;
   LLVM_DEBUG(dbgs() << "SLP: Calculating cost for tree of size "
                     << VectorizableTree.size() << ".\n");