nir/linker: Add a pure NIR implementation of the atomic counter linker

nir/linker: Add a pure NIR implementation of the atomic counter linker

This is mostly just a straight-forward conversion of
link_assign_atomic_counter_resources to C directly using nir variables
instead of GLSL IR variables.

It is based on the version of link_assign_atomic_counter_resources in
6b8909f2d1906. I’m noting this here to make it easier to track changes
and keep the NIR version up-to-date.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>