Intergroup Randomized Phase III Study of Postoperative Irinotecan; 5-Fluorouracil and Leucovorin Vs Oxaliplatin; 5-Fluorouracil and Leucovorin Vs 5-Fluorouracil and Leucovorin For Patients with Stage II or III Rectal Cancer Receiving Either Preoperative Radiation and 5-Fluorouracil or Postoperative Radiation and 5-Fluorouracil