nir/deref: Skip over casts in fixup_deref_modes

This pass is used when, for instance, we lazily change the mode of
variables rather than replacing the variable with a new one.  Since we
only do this in cases where we know we have full deref chains, it's ok
to just skip them in fixup_deref_modes.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/compiler/nir/nir_deref.c

@@ -272,6 +272,8 @@ nir_fixup_deref_modes(nir_shader *shader)
                continue;
 
             nir_deref_instr *deref = nir_instr_as_deref(instr);
+            if (deref->deref_type == nir_deref_type_cast)
+               continue;
 
             nir_variable_mode parent_mode;
             if (deref->deref_type == nir_deref_type_var) {