Implements AIR lowering for is_null and is_non_null tags.

Additionally this cleans up and centralizes the logic to convert from 'direct'
representation to 'indirect' representation and vice-versa. The related functions,
as well as the functions that use it, are all moved near eachother so that the
conversion logic remains in a central place. Extracting/inserting fields
and loading/storing pointers should go through these functions.