[pseudo] Store last node popped in the queue, not its parent(s). NFC

We have to walk up to the last node to find the start token, but no need
to go even one node further.

This is one node fewer to store, but more importantly if the last node
happens to have multiple parents we avoid storing the sequence multiple times.

This saves ~5% on glrParse.
Based on a comment by hokein@ on https://reviews.llvm.org/D128307

Author: sam-mccall

clang-tools-extra/pseudo/lib/GLR.cpp

@@ -215,8 +215,9 @@ class GLRReduce {
   // They specify a sequence ForestNode we may build (but we dedup first).
   // (The RuleID is not stored here, but rather in the Family).
   struct PushSpec {
-    // A base node is the head after popping the GSS nodes we are reducing.
-    const GSS::Node* Base = nullptr;
+    // The last node popped before pushing. Its parent is the reduction base(s).
+    // (Base is more fundamental, but this is cheaper to store).
+    const GSS::Node* LastPop = nullptr;
     Sequence *Seq = nullptr;
   };
   KeyedQueue<Family, PushSpec> Sequences; // FIXME: rename => PendingPushes?
@@ -256,9 +257,13 @@ class GLRReduce {
     Family F{/*Start=*/0, /*Symbol=*/Rule.Target, /*Rule=*/RID};
     TempSequence.resize_for_overwrite(Rule.Size);
     auto DFS = [&](const GSS::Node *N, unsigned I, auto &DFS) {
-      if (I == Rule.Size) {
+      TempSequence[Rule.Size - 1 - I] = N->Payload;
+      if (I + 1 == Rule.Size) {
         F.Start = TempSequence.front()->startTokenIndex();
-        LLVM_DEBUG(llvm::dbgs() << "    --> base at S" << N->State << "\n");
+        LLVM_DEBUG({
+          for (const auto *B : N->parents())
+            llvm::dbgs() << "    --> base at S" << B->State << "\n";
+        });
 
         // Copy the chain to stable storage so it can be enqueued.
         if (SequenceStorageCount == SequenceStorage.size())
@@ -269,7 +274,6 @@ class GLRReduce {
         Sequences.emplace(F, PushSpec{N, Seq});
         return;
       }
-      TempSequence[Rule.Size - 1 - I] = N->Payload;
       for (const GSS::Node *Parent : N->parents())
         DFS(Parent, I + 1, DFS);
     };
@@ -313,12 +317,11 @@ class GLRReduce {
     FamilySequences.clear();
     FamilyBases.clear();
     do {
-      FamilySequences.emplace_back(Sequences.top().first.Rule,
-                                   *Sequences.top().second.Seq);
-      FamilyBases.emplace_back(
-          Params.Table.getGoToState(Sequences.top().second.Base->State,
-                                    F.Symbol),
-          Sequences.top().second.Base);
+      const PushSpec &Push = Sequences.top().second;
+      FamilySequences.emplace_back(Sequences.top().first.Rule, *Push.Seq);
+      for (const GSS::Node *Base : Push.LastPop->parents())
+        FamilyBases.emplace_back(
+            Params.Table.getGoToState(Base->State, F.Symbol), Base);
 
       Sequences.pop();
     } while (!Sequences.empty() && Sequences.top().first == F);