nir/algebraic: Lower CS derivatives to zero when no group defined

nir/algebraic: Lower CS derivatives to zero when no group defined

In compute shaders if no derivative group is defined, the derivatives
will always be zero.  Specified in NV_compute_shader_derivatives.

To make the check more convenient, add a "info" local variable to the
generated code so we can refer to it in the Python rules.  (Jason)

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>