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/compiler/nir/tests/negative_equal_tests.cpp

@@ -57,7 +57,7 @@ protected:
       glsl_type_singleton_init_or_ref();
 
       static const nir_shader_compiler_options options = { };
-      nir_builder_init_simple_shader(&bld, NULL, MESA_SHADER_VERTEX, &options);
+      bld = nir_builder_init_simple_shader(NULL, MESA_SHADER_VERTEX, &options);
       memset(c1, 0, sizeof(c1));
       memset(c2, 0, sizeof(c2));
    }