A 60-year-old woman noticed an enlarging "bump" beneath her tongue for the past year. She does not smoke or use alcohol. On physical examination, there is a 2.5-cm, movable, submucosal mass arising in the minor salivary glands on the buccal mucosa beneath the tongue on the right. Histologic examination of the excised mass shows that it is malignant and locally invasive. The tumor recurs within 1 year. Which of the following is the most likely diagnosis?