Fix the OP_ID of OpAssignGroupNodes

Commit 96276ae7 added the above opcode, but forgot to update the OP_ID
per our previous mailing list discussions. This patch makes the OP_ID
be consistent with the other IDs (OP_$object_$action).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>