If*** can use their query expression counterparts

[bserdar]

For instance, IfFieldCheckField declares a field, op, and rfield. Then it deals with to-from string translation, parsing, etc. All these are already implemented in QueryExpression classes, and those should be used instead. That is, in IfFieldCheckField, instead of declaring field, rfield, and op, declare a FieldComparisonExpression. IfFieldCheckValue can use ValueComparisonExpression, IfFieldCheckValues can use NaryRelationalExpression, regex can use RegexExpression...etc.