[Mesa-dev] radeonsi add nir support for shared vars/atomics

[Timothy Arceri]

This adds the missing nir support and allows the nir backend to produce
an equivalent shader to the tgsi backend for the only piglit test we
have for this [1]. However the test still fails and I've so far been
unable to spot the problem.

[1] ./bin/shader_runner tests/spec/arb_compute_shader/execution/shared-atomics.shader_test -auto