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.