SAN JUAN (Reuters) - Puerto Rico held its Republican presidential primary on Sunday, with 20 of the U.S. territory's 23 delegates up for grabs in a race in which an upcoming vote on statehood has taken central stage. Mitt Romney, campaigning hard for the trove of delegates on the island, had initially planned to stay in San Juan until Sunday before heading to Illinois, which holds a crucial primary on Tuesday. But the Republican front-runner decided Friday to return to Illinois a day earlier than scheduled in a sign of the urgency his campaign feels to win Illinois over rival Rick Santorum. ...