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

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.