st/mesa: fix crash for drivers supporting nir defaulting to tgsi

st/mesa: fix crash for drivers supporting nir defaulting to tgsi

nvc0 and I assume radeonsi as well hit an assert inside glsl_to_tgsi as atan
instructions get inserted into the shader.

Fixes: cece947a8dfa ("glsl/builtin: Add alternate versions of atan using new ops")
Cc: Neil Roberts <nroberts@igalia.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Rob Clark <robdclark@gmail.com>