Loews said it raised its stake in CBS Inc. to 17.7% from 17% by purchasing 4,169,921 shares at prices ranging from $131 to $134.37 1/2 per share . . . Great Atlantic & Pacific Tea Co. said it agreed to acquire Shopwell, the New York-based supermarket chain, for $31 a share, or $64 million . . . Western Airlines said it flew 935 million revenue passenger miles in May, down from 940 million in May, 1985.