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

{
        if (2 != arguments.size())
            return false;
        return castBothTypes(arguments[0].get(), arguments[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>)
                return false;
            else
            {
                using ResultDataType = typename BinaryOperationTraits<Op, LeftDataType, RightDataType>::ResultDataType;
                using OpSpec = Op<typename LeftDataType::FieldType, typename RightDataType::FieldType>;
                return !std::is_same_v<ResultDataType, InvalidType> && !IsDataTypeDecimal<ResultDataType> && OpSpec::compilable;
            }
        });
    }
    llvm::Value * compileImpl(llvm::IRBuilderBase & builder, const DataTypes & types, ValuePlaceholders values) const override