Fix UnusedValueSlotRemover such that it doesn't eliminate outer references

[terrajobst]

For example, in this snippet

SELECT  (
            SELECT  COUNT(*)
            FROM    EmployeeTerritories et
            WHERE   et.EmployeeID = e.EmployeeID
        ) AS TerritoryCount
FROM    Employees e

the table scan for Employees no longer included e.EmployeeID.