nir: Consider atomic counter intrinsics when setting writes_memory

In i965 these get lowered after gather info, so let's consider them too. Fixes

piglit.spec.arb_framebuffer_no_attachments.arb_framebuffer_no_attachments-atomic

in Gen9, HSW and IVB.

Fixes: 6a6c36e9 ("intel/fs: Use writes_memory from shader_info")