nir/lower_io_to_vector: don't merge compact varyings

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Fixes: 02bc4aabb48 ('nir/lower_io_to_vector: allow FS outputs to be vectorized')
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/compiler/nir/nir_lower_io_to_vector.c

@@ -81,6 +81,9 @@ variables_can_merge(const nir_shader *shader,
                     const nir_variable *a, const nir_variable *b,
                     bool same_array_structure)
 {
+   if (a->data.compact || b->data.compact)
+      return false;
+
    const struct glsl_type *a_type_tail = a->type;
    const struct glsl_type *b_type_tail = b->type;