Ironwood Pharmaceuticals Inc., maker of a constipation treatment, said its quarterly loss widened as higher expenses outweighed revenue gains. Ironwood lost $18.8 million, or 19 cents per share. Revenue rose 23 percent to $11.3 million. The Cambridge company said operating expenses rose 14 percent. Ironwood has submitted linaclotide, its potential treatment for irritable bowel syndrome and chronic constipation, to regulators for approval.