nir: Add pass to lower image atomics

nir: Add pass to lower image atomics

Hardware that lacks dedicated image atomics can still implement image atomics
with regular atomics on global memory, as long as there is a way to get the
address of a texel in memory. I've open-coded this lowering in my first 2
compilers, so before I add another crappy vendored version in my 3rd, let's add
a common NIR pass to do the lowering.

Thanks to unified atomics, the pass itself is fairly concise.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23120>