aco/ngg: Make primitive export packing less prone to error.

Use lshl_or instead of lshl_add, which makes it more robust in
handling -1 and -2 indices which will now just become null
exports, which is what we want.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <mesa/mesa!6964>