turnip: Add nir_opt_conditional_discard.

We can easily do discard_if in the backend without control flow, but it
wasn't done in ir3 because the GL frontend already did it for us.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15982>

src/freedreno/vulkan/tu_shader.c

@@ -132,6 +132,8 @@ tu_spirv_to_nir(struct tu_device *dev,
 
    ir3_optimize_loop(dev->compiler, nir);
 
+   NIR_PASS_V(nir, nir_opt_conditional_discard);
+
    return nir;
 }