tu: Make sure we clear dead writes to vars before nir_link_opt_varyings().

It assumes you don't have dead writes to variables in the last block, and
will copy-propagate consts from the first write it finds.

Without this, the upcoming nir_opt_copy_prop_vars() change to have more
restricted write masks caused less nir_opt_dead_write_vars() (since it
doesn't trim write masks for dead writes, only removes fully-dead writes),
and then zero-initialization of variables at the top of a shader got
propagated, rather than the final store of the used channels of the
variable.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37313>

src/freedreno/vulkan/tu_shader.cc

@@ -2923,6 +2923,12 @@ lower_io_to_scalar_early(nir_shader *nir, nir_variable_mode mask)
        */
       NIR_PASS(_, nir, nir_opt_copy_prop_vars);
 
+      /* This must be called before nir_link_opt_varyings() and after
+       * nir_opt_copy_prop_vars(), otherwise repeated (scalarized) stores in the
+       * last block will propagate the wrong values into the consumer.
+       */
+      NIR_PASS(_, nir, nir_opt_dead_write_vars);
+
       NIR_PASS(_, nir, nir_opt_dce);
 
       const nir_remove_dead_variables_options var_opts = {