Currently, st/mesa is always calling the GLSL IR lower_instructions()
pass with MOD_TO_FLOOR set, so mod operations will be lowered before
ever reaching NIR.  This enables the same lowering at the NIR level,
which will let me shut off the GLSL IR path for NIR-based drivers.

The AMD NIR backend also has code to handle fmod, so we could
potentially skip this and still be fine.  I don't have an opinion
on that.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>