[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

@@ -15777,6 +15777,8 @@ static bool compareCmp(Value *V, Value *V2, TargetLibraryInfo &TLI,
           if (NodeI1 != NodeI2)
             return NodeI1->getDFSNumIn() < NodeI2->getDFSNumIn();
         }
+        if (I1->getOpcode() == I2->getOpcode())
+          continue;
         InstructionsState S = getSameOpcode({I1, I2}, TLI);
         if (S.getOpcode() && (IsCompatibility || !S.isAltShuffle()))
           continue;