Push down raw values to storage in filter

The following filter won't be pushed down to storage.

```
filter(fn: (r) => r["_value"])
```

In general, this isn't a valid query that returns a boolean because `_value` is usually not a boolean. But, when `_value` is a boolean, this filter is valid and should be pushed down just like:

```
filter(fn: (r) => r["_value"] == true)
```

Because of the difference in the planner, these queries perform at vastly different speeds. We should address this difference. The planner should be able to push down the first one and storage can reject the filter if `_value` is not a boolean value.

No file or test entry point is named. Start by tracing how the planner handles a filter on raw _value and how storage receives pushed-down filters. Verify that boolean _value filters using the direct expression behave like comparisons with true, while non-boolean values can still be rejected by storage.

Written by the indexing model from the issue text.