turnip: Execute ir3_nir_lower_gs pass again

turnip: Execute ir3_nir_lower_gs pass again

This commit fixes a GS regression introduced in !4562 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.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4892>