nir: Make "divergent" a property of an SSA value

v2: fix usage in ACO (by Daniel Schürmann)

Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4062>

src/compiler/nir/nir.c

@@ -1451,6 +1451,7 @@ nir_ssa_def_init(nir_instr *instr, nir_ssa_def *def,
    list_inithead(&def->if_uses);
    def->num_components = num_components;
    def->bit_size = bit_size;
+   def->divergent = true; /* This is the safer default */
 
    if (instr->block) {
       nir_function_impl *impl =