I've been studying basic usage and I found wrong minus result on DecimalXX type.

**Example Query**
`
select toDecimal64(4,4) - toDecimal32(2,2)
`

**Expected output**
2.0000

**Actual output**
3.9800

Is there anything wrong or maybe my misunderstanding of toDecimal ?

![image](https://user-images.githubusercontent.com/8475371/107513855-cdd57a00-6be3-11eb-841d-d425a198d172.png)


**How to reproduce**
1. OS: macOS Catalina 10.15.5
2. server: clickhouse-server, built from source, version 21.3.1.1
3. client:  clickhouse-client,  built from source, version 21.3.1.1

{
        assert(2 == types.size() && 2 == values.size());
        llvm::Value * result = nullptr;
        castBothTypes(types[0].get(), types[1].get(), [&](const auto & left, const auto & right)
        {
            using LeftDataType = std::decay_t<decltype(left)>;
            using RightDataType = std::decay_t<decltype(right)>;
            if constexpr (!std::is_same_v<DataTypeFixedString, LeftDataType> && !std::is_same_v<DataTypeFixedString, RightDataType>)
            {
                using ResultDataType = typename BinaryOperationTraits<Op, LeftDataType, RightDataType>::ResultDataType;
                using OpSpec = Op<typename LeftDataType::FieldType, typename RightDataType::FieldType>;
                if constexpr (!std::is_same_v<ResultDataType, InvalidType> && !IsDataTypeDecimal<ResultDataType> && OpSpec::compilable)
                {
                    auto & b = static_cast<llvm::IRBuilder<> &>(builder);
                    auto type = std::make_shared<ResultDataType>();
                    auto * lval = nativeCast(b, types[0], values[0](), type);
                    auto * rval = nativeCast(b, types[1], values[1](), type);
                    result = OpSpec::compile(b, lval, rval, std::is_signed_v<typename ResultDataType::FieldType>);
                    return true;
                }
            }
            return false;
        });
        return result;
    }
#endif
    bool canBeExecutedOnDefaultArguments() const override { return valid_on_default_arguments; }
};
template <template <typename, typename> class Op, typename Name, bool valid_on_default_arguments = true, bool valid_on_float_arguments = true>
class FunctionBinaryArithmeticWithConstants : public FunctionBinaryArithmetic<Op, Name, valid_on_default_arguments, valid_on_float_arguments>