Morgan Guaranty, the nation's fifth-largest bank, raised its prime lending rate to 20 percent from 19 1/2 percent. The move was promptly matched by First National Bank of Chicago. The prime rate is now at its highest level since Jan. 5, when rates temporarily declined before heading back up last month. The action came after Friday's Federal Reserve report that the nation's money supply jumped $3.1 billion in the latest week. While the figure was lower than anticipated, analysts said it was still seen as abnormal and expressed fu rther concern over a recent jump in credit loan demand.