i965/nir: Run DCE again before going out of SSA

We run lowering and optimization passes that might leave garbage lying
around. This keeps the FS cse from having to clean it up.

Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_fs_nir.cpp

@@ -163,6 +163,8 @@ fs_visitor::emit_nir_code()
    nir_validate_shader(nir);
    nir_copy_prop(nir);
    nir_validate_shader(nir);
+   nir_opt_dce(nir);
+   nir_validate_shader(nir);
 
    if (unlikely(debug_enabled)) {
       fprintf(stderr, "NIR (SSA form) for %s shader:\n", stage_name);