[SLP] NFC. Remove the useless check for alternate instruction. (#117116)

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

Author: HanKuanChen

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -975,6 +975,9 @@ static InstructionsState getSameOpcode(ArrayRef<Value *> VL,
       Type *Ty1 = Inst->getOperand(0)->getType();
       if (Ty0 == Ty1) {
         assert(InstOpcode == Opcode && "Expected same CmpInst opcode.");
+        assert(InstOpcode == AltOpcode &&
+               "Alternate instructions are only supported by BinaryOperator "
+               "and CastInst.");
         // Check for compatible operands. If the corresponding operands are not
         // compatible - need to perform alternate vectorization.
         CmpInst::Predicate CurrentPred = Inst->getPredicate();
@@ -1003,7 +1006,10 @@ static InstructionsState getSameOpcode(ArrayRef<Value *> VL,
             AltPred == CurrentPred || AltPred == SwappedCurrentPred)
           continue;
       }
-    } else if (InstOpcode == Opcode || InstOpcode == AltOpcode) {
+    } else if (InstOpcode == Opcode) {
+      assert(InstOpcode == AltOpcode &&
+             "Alternate instructions are only supported by BinaryOperator and "
+             "CastInst.");
       if (auto *Gep = dyn_cast<GetElementPtrInst>(I)) {
         if (Gep->getNumOperands() != 2 ||
             Gep->getOperand(0)->getType() != IBase->getOperand(0)->getType())
@@ -5999,8 +6005,10 @@ void BoUpSLP::reorderTopToBottom() {
       if ((TE->State == TreeEntry::Vectorize ||
            TE->State == TreeEntry::StridedVectorize) &&
           isa<ExtractElementInst, ExtractValueInst, LoadInst, StoreInst,
-              InsertElementInst>(TE->getMainOp()) &&
-          !TE->isAltShuffle()) {
+              InsertElementInst>(TE->getMainOp())) {
+        assert(!TE->isAltShuffle() &&
+               "Alternate instructions are only supported by BinaryOperator "
+               "and CastInst.");
         // Build correct orders for extract{element,value}, loads and
         // stores.
         reorderOrder(TE->ReorderIndices, Mask);