Irish medical supplies company Covidien PLC, which employs about 1,800 people in Massachusetts, said it has agreed to buy Israel's Given Imaging Ltd. as part of its plan to expand into the multibillion dollar global gastrointestinal market. The deal is valued at about $860 million. Given Imaging's product line includes PillCam, a small endoscope that can be swallowed in a capsule. Covidien, which has its US headquarters in Mansfield, has recently made several smaller acquisitions of firms that specialize in gastrointestinal treatments.