Revert "[SLP]Improve findReusedOrderedScalars processing, NFCI."

This reverts commit 44dc1e0.

Causes non-determinism, see #75987.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -3760,87 +3760,61 @@ BoUpSLP::findReusedOrderedScalars(const BoUpSLP::TreeEntry &TE) {
   OrdersType CurrentOrder(NumScalars, NumScalars);
   SmallVector<int> Positions;
   SmallBitVector UsedPositions(NumScalars);
-  DenseMap<const TreeEntry *, unsigned> UsedEntries;
-  DenseMap<Value *, std::pair<const TreeEntry *, unsigned>> ValueToEntryPos;
-  for (Value *V : TE.Scalars) {
-    if (!isa<LoadInst, ExtractElementInst, ExtractValueInst>(V))
-      continue;
-    const auto *LocalSTE = getTreeEntry(V);
-    if (!LocalSTE)
-      continue;
-    unsigned Lane =
-        std::distance(LocalSTE->Scalars.begin(), find(LocalSTE->Scalars, V));
-    if (Lane >= NumScalars)
-      continue;
-    ++UsedEntries.try_emplace(LocalSTE, 0).first->getSecond();
-    ValueToEntryPos.try_emplace(V, LocalSTE, Lane);
-  }
-  if (UsedEntries.empty())
-    return std::nullopt;
-  const TreeEntry &BestSTE =
-      *std::max_element(UsedEntries.begin(), UsedEntries.end(),
-                        [](const std::pair<const TreeEntry *, unsigned> &P1,
-                           const std::pair<const TreeEntry *, unsigned> &P2) {
-                          return P1.second < P2.second;
-                        })
-           ->first;
-  UsedEntries.erase(&BestSTE);
-  const TreeEntry *SecondBestSTE = nullptr;
-  if (!UsedEntries.empty())
-    SecondBestSTE =
-        std::max_element(UsedEntries.begin(), UsedEntries.end(),
-                         [](const std::pair<const TreeEntry *, unsigned> &P1,
-                            const std::pair<const TreeEntry *, unsigned> &P2) {
-                           return P1.second < P2.second;
-                         })
-            ->first;
+  const TreeEntry *STE = nullptr;
   // Try to find all gathered scalars that are gets vectorized in other
   // vectorize node. Here we can have only one single tree vector node to
   // correctly identify order of the gathered scalars.
   for (unsigned I = 0; I < NumScalars; ++I) {
     Value *V = TE.Scalars[I];
     if (!isa<LoadInst, ExtractElementInst, ExtractValueInst>(V))
       continue;
-    const auto [LocalSTE, Lane] = ValueToEntryPos.lookup(V);
-    if (!LocalSTE || (LocalSTE != &BestSTE && LocalSTE != SecondBestSTE))
-      continue;
-    if (CurrentOrder[Lane] != NumScalars) {
-      if ((CurrentOrder[Lane] >= BestSTE.Scalars.size() ||
-           BestSTE.Scalars[CurrentOrder[Lane]] == V) &&
-          (Lane != I || LocalSTE == SecondBestSTE))
-        continue;
-      UsedPositions.reset(CurrentOrder[Lane]);
+    if (const auto *LocalSTE = getTreeEntry(V)) {
+      if (!STE)
+        STE = LocalSTE;
+      else if (STE != LocalSTE)
+        // Take the order only from the single vector node.
+        return std::nullopt;
+      unsigned Lane =
+          std::distance(STE->Scalars.begin(), find(STE->Scalars, V));
+      if (Lane >= NumScalars)
+        return std::nullopt;
+      if (CurrentOrder[Lane] != NumScalars) {
+        if (Lane != I)
+          continue;
+        UsedPositions.reset(CurrentOrder[Lane]);
+      }
+      // The partial identity (where only some elements of the gather node are
+      // in the identity order) is good.
+      CurrentOrder[Lane] = I;
+      UsedPositions.set(I);
     }
-    // The partial identity (where only some elements of the gather node are
-    // in the identity order) is good.
-    CurrentOrder[Lane] = I;
-    UsedPositions.set(I);
   }
   // Need to keep the order if we have a vector entry and at least 2 scalars or
   // the vectorized entry has just 2 scalars.
-  if (BestSTE.Scalars.size() != 2 && UsedPositions.count() <= 1)
-    return std::nullopt;
-  auto IsIdentityOrder = [&](ArrayRef<unsigned> CurrentOrder) {
-    for (unsigned I = 0; I < NumScalars; ++I)
-      if (CurrentOrder[I] != I && CurrentOrder[I] != NumScalars)
-        return false;
-    return true;
-  };
-  if (IsIdentityOrder(CurrentOrder))
-    return OrdersType();
-  auto *It = CurrentOrder.begin();
-  for (unsigned I = 0; I < NumScalars;) {
-    if (UsedPositions.test(I)) {
-      ++I;
-      continue;
-    }
-    if (*It == NumScalars) {
-      *It = I;
-      ++I;
+  if (STE && (UsedPositions.count() > 1 || STE->Scalars.size() == 2)) {
+    auto &&IsIdentityOrder = [NumScalars](ArrayRef<unsigned> CurrentOrder) {
+      for (unsigned I = 0; I < NumScalars; ++I)
+        if (CurrentOrder[I] != I && CurrentOrder[I] != NumScalars)
+          return false;
+      return true;
+    };
+    if (IsIdentityOrder(CurrentOrder))
+      return OrdersType();
+    auto *It = CurrentOrder.begin();
+    for (unsigned I = 0; I < NumScalars;) {
+      if (UsedPositions.test(I)) {
+        ++I;
+        continue;
+      }
+      if (*It == NumScalars) {
+        *It = I;
+        ++I;
+      }
+      ++It;
     }
-    ++It;
+    return std::move(CurrentOrder);
   }
-  return std::move(CurrentOrder);
+  return std::nullopt;
 }
 
 namespace {