Function skipValueInConditionalExpression

* Should value be skipped because it's hidden inside && || or ?: expression. * Example: ((x!=NULL) && (*x == 123)) * If 'valuetok' points at the x in '(*x == 123)'. Then the '&&' will be returned. * @param valuetok original variable token * @return NULL=>don't skip, non-NULL=>The operator token that cause the skip. For instance the '&&'. * */