nir_builder: Return a new builder from nir_builder_init_simple_shader().

It's a little inline function, so we can just RAII it for better
ergonomics.

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7323>

src/intel/compiler/brw_nir.c

@@ -1475,9 +1475,8 @@ brw_nir_create_passthrough_tcs(void *mem_ctx, const struct brw_compiler *compile
                                const nir_shader_compiler_options *options,
                                const struct brw_tcs_prog_key *key)
 {
-   nir_builder b;
-   nir_builder_init_simple_shader(&b, mem_ctx, MESA_SHADER_TESS_CTRL,
-                                  options);
+   nir_builder b = nir_builder_init_simple_shader(mem_ctx, MESA_SHADER_TESS_CTRL,
+                                                  options);
    nir_shader *nir = b.shader;
    nir_variable *var;
    nir_intrinsic_instr *load;