st/nir: Don't use GLSL IR's MOD_TO_FLOOR lowering when using NIR.

Both GLSL IR and NIR perform the same mod -> floor lowering for 32-bit
types.  But nir_lower_double_ops is slightly more defensive against
lowered drcp precision loss, and handles mod(x, x) = 0 directly.  This
works well...assuming nir_lower_double_ops actually gets an fmod op to
lower in the first place.

The previous patches enabled NIR-based lowering for the remaining
drivers, so we can stop using the GLSL IR lowering when using NIR.

Fixes KHR-GL45.gpu_shader_fp64.builtin.mod_dvec[234] on iris.

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