compiler/nir: add a divergence analysis option for non-uniform workgroup id

compiler/nir: add a divergence analysis option for non-uniform workgroup id

The V3D hardware allows us to pack multiple workgroups together to avoid
wasting execution lanes in shader cores.

For example, if we dispatch 16 workgroups with a local size of 1 element, we
can pack all 16 workgroups in a single 16-wide dispatch where each lane
executes a different workgroup, instead of 16 1-wide dispatches.

When we do this, we don't have a uniform workgroup id any more.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10541>