Optimize truncate, ceiling and friends when divisor is 1 or -1.

Optimize truncate, ceiling and friends when divisor is 1 or -1.

If divisor is 1, return (values x 0), if it's -1 (values (- x) 0). If
it's a floating point truncation, like ftruncate or fceiling,
(values (- (float x)) 0).