[SLP] NFC. Remove the useless check for alternate instruction. (#117293)

Only BinaryOperator and CastInst support alternate instruction. It
always returns false for TreeEntry::isAltShuffle if an instruction is
ExtractElementInst, ExtractValueInst, LoadInst, StoreInst or
InsertElementInst.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -5477,11 +5477,13 @@ BoUpSLP::getReorderingData(const TreeEntry &TE, bool TopToBottom) {
     // Try build correct order for extractelement instructions.
     SmallVector<int> ReusedMask(TE.ReuseShuffleIndices.begin(),
                                 TE.ReuseShuffleIndices.end());
-    if (TE.getOpcode() == Instruction::ExtractElement && !TE.isAltShuffle() &&
+    if (TE.getOpcode() == Instruction::ExtractElement &&
         all_of(TE.Scalars, [Sz](Value *V) {
           std::optional<unsigned> Idx = getExtractIndex(cast<Instruction>(V));
           return Idx && *Idx < Sz;
         })) {
+      assert(!TE.isAltShuffle() && "Alternate instructions are only supported "
+                                   "by BinaryOperator and CastInst.");
       SmallVector<int> ReorderMask(Sz, PoisonMaskElem);
       if (TE.ReorderIndices.empty())
         std::iota(ReorderMask.begin(), ReorderMask.end(), 0);
@@ -5526,9 +5528,11 @@ BoUpSLP::getReorderingData(const TreeEntry &TE, bool TopToBottom) {
   if ((TE.State == TreeEntry::Vectorize ||
        TE.State == TreeEntry::StridedVectorize) &&
       (isa<LoadInst, ExtractElementInst, ExtractValueInst>(TE.getMainOp()) ||
-       (TopToBottom && isa<StoreInst, InsertElementInst>(TE.getMainOp()))) &&
-      !TE.isAltShuffle())
+       (TopToBottom && isa<StoreInst, InsertElementInst>(TE.getMainOp())))) {
+    assert(!TE.isAltShuffle() && "Alternate instructions are only supported by "
+                                 "BinaryOperator and CastInst.");
     return TE.ReorderIndices;
+  }
   if (TE.State == TreeEntry::Vectorize && TE.getOpcode() == Instruction::PHI) {
     if (!TE.ReorderIndices.empty())
       return TE.ReorderIndices;
@@ -5924,8 +5928,11 @@ void BoUpSLP::reorderTopToBottom() {
           continue;
       }
       // Stores actually store the mask, not the order, need to invert.
-      if (OpTE->State == TreeEntry::Vectorize && !OpTE->isAltShuffle() &&
+      if (OpTE->State == TreeEntry::Vectorize &&
           OpTE->getOpcode() == Instruction::Store && !Order.empty()) {
+        assert(!OpTE->isAltShuffle() &&
+               "Alternate instructions are only supported by BinaryOperator "
+               "and CastInst.");
         SmallVector<int> Mask;
         inversePermutation(Order, Mask);
         unsigned E = Order.size();
@@ -6188,8 +6195,11 @@ void BoUpSLP::reorderBottomToTop(bool IgnoreReorder) {
               return P.second == OpTE;
             });
         // Stores actually store the mask, not the order, need to invert.
-        if (OpTE->State == TreeEntry::Vectorize && !OpTE->isAltShuffle() &&
+        if (OpTE->State == TreeEntry::Vectorize &&
             OpTE->getOpcode() == Instruction::Store && !Order.empty()) {
+          assert(!OpTE->isAltShuffle() &&
+                 "Alternate instructions are only supported by BinaryOperator "
+                 "and CastInst.");
           SmallVector<int> Mask;
           inversePermutation(Order, Mask);
           unsigned E = Order.size();