turnip: Execute ir3_nir_lower_gs pass again

This commit fixes a GS regression introduced in !4562 (merged) where ir3's GS lowering pass was moved from common code (ir3_nir) to freedreno-specific code (ir3_shader). For GS support in turnip, we need to add the GS lowering pass back in, this time in tu_shader.

As for the nir_gather_info change, the GS lowering pass has always introduced a discard_if intrinsic into the GS. Previously, we simply ran nir_shader_gather_info before GS lowering, but now since we lower the GS before we need to remove the assertion that only a FS can use the discard_if intrinsic.