There's no single instruction that I know of, but you could use indirect addressing and a counter to examine each array entry and compare it with a "current lowest" value, replacing if the examined value is lower than the compared value.
So, whenever you want to initiate an update to the highest/lowest, reset the counter to zero and use it's accumulator as the indirect variable.
IF Array[counter.ACC] < CurrentLowestValue THEN MOV Array[counter.ACC] TO CurrentLowestValue
IF Array[counter.ACC] > CurrentHighestValue THEN MOV Array[coutner.ACC] TO CurrentHighestValue
Then increment the counter and do it again. Make the counter preset equal to the length of your array, and when the counter DN bit turns on, reset the counter. Will be ready to compare again whenever you flag it again.