nir/lower_ssbo: handle atomics

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Tested-by: Marge Bot <mesa/mesa!2753>
Part-of: <mesa/mesa!2753>
40 jobs for !2753 with just-nir-lower in 46 minutes and 48 seconds (queued for 7 seconds)
merge request