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 on iris. Reviewed-by: Marek Olšák <firstname.lastname@example.org>