planner: fix the wrong cost formula of MPPExchanger on cost model ver2 (#35718)
planner: fix the wrong cost formula of MPPExchanger on cost model ver2 (#35718)

ref pingcap/tidb#35240
kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,pingcap/tidb,kyle-wang/tidb,ngaut/tidb