[SLP][NFC]Check for equal opcode preliminary to meet weak strict order

requirement, NFC.

This change does not affect functionality, just fixes the assertions in
some standard c++ library implementations.

Author: alexey-bataev

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -15730,6 +15730,8 @@ static bool compareCmp(Value *V, Value *V2, TargetLibraryInfo &TLI,
   assert(isValidElementType(V->getType()) &&
          isValidElementType(V2->getType()) &&
          "Expected valid element types only.");
+  if (V == V2)
+    return IsCompatibility;
   auto *CI1 = cast<CmpInst>(V);
   auto *CI2 = cast<CmpInst>(V2);
   if (CI1->getOperand(0)->getType()->getTypeID() <
@@ -15754,6 +15756,8 @@ static bool compareCmp(Value *V, Value *V2, TargetLibraryInfo &TLI,
   for (int I = 0, E = CI1->getNumOperands(); I < E; ++I) {
     auto *Op1 = CI1->getOperand(CI1Preds ? I : E - I - 1);
     auto *Op2 = CI2->getOperand(CI2Preds ? I : E - I - 1);
+    if (Op1 == Op2)
+      continue;
     if (Op1->getValueID() < Op2->getValueID())
       return !IsCompatibility;
     if (Op1->getValueID() > Op2->getValueID())
@@ -15780,7 +15784,10 @@ static bool compareCmp(Value *V, Value *V2, TargetLibraryInfo &TLI,
         InstructionsState S = getSameOpcode({I1, I2}, TLI);
         if (S.getOpcode() && (IsCompatibility || !S.isAltShuffle()))
           continue;
-        return !IsCompatibility && I1->getOpcode() < I2->getOpcode();
+        if (IsCompatibility)
+          return false;
+        if (I1->getOpcode() != I2->getOpcode())
+          return I1->getOpcode() < I2->getOpcode();
       }
   }
   return IsCompatibility;