Optimization: OutputPlan should only skip AccessStep if it could be tree-shaken

[benjie]

Currently all OutputPlan that reference a suitable AccessStep will skip the access step and use the expression hoping that it can be optimized away. Instead, we should only do this if the AccessStep can be tree shaken away (ignoring OutputPlans) - if not, since we've already calculated it we should have the OutputPlan use the precalculated values.