[GISel][CombinerHelper] Use uses() instead of operands() when traversing use operands.

  // Walk over all the operands of concat vectors and check if they are

  // build_vector themselves or undef.

  // Then collect their operands in Ops.

  for (const MachineOperand &MO : MI.operands()) {

    // Skip the instruction definition.

    if (MO.isDef())

      continue;

  for (const MachineOperand &MO : MI.uses()) {

    Register Reg = MO.getReg();

    MachineInstr *Def = MRI.getVRegDef(Reg);

    assert(Def && "Operand not defined");

      IsUndef = false;

      // Remember the operands of the build_vector to fold

      // them into the yet-to-build flattened concat vectors.

      for (const MachineOperand &BuildVecMO : Def->operands()) {

        // Skip the definition.

        if (BuildVecMO.isDef())

          continue;

      for (const MachineOperand &BuildVecMO : Def->uses())

        Ops.push_back(BuildVecMO.getReg());

      }

      break;

    case TargetOpcode::G_IMPLICIT_DEF: {

      LLT OpType = MRI.getType(Reg);