7121073: secondary_super_cache memory slice has incorrect bounds in flatten_alias_type
Reviewed-by:

To ensure that memory operations for type checks are properly ordered,
several fields are smeared together into the same alias slice. The
existing logic is sloppy and includes a slightly larger chunk of
memory than intended. Currently this is fine but changes to the shape
of Klass can invalidate it. The fix is to properly test the
boundaries. Additionally I changed the logic for setting the PTR to
use NotNull instead of Constant to be consisten with all the following
logic which uses NotNull. Tested with full CTW.