Fix: todos in datagraph and nodeop replacement bug

[Unparalleled-Calvin]

• Some match arms in datagraph have been implemented now to avoid panic
• Change the basic field op in the GraphNode.
  • Previously, our datagraph was build upon a hypothesis that every Local in MIR should be assigned for once and only once.
  • While actually we found that a Local can be assigned for multiple times. Thus I added the seq marker in edges to differentiate between edges.
  • However, the nodeop also needs to save the previous one and keep every record available.
  • In this PR, the op field is replaced by ops which is a Vec<NodeOp> and corresponding codes are also revised.