math/rand: remove noop iteration in Perm

The first iteration always do `m[0], m[0] = m[0], m[0]`, because
`rand.Intn(1)` is 0.

fun note: IIRC in TAOCP version of this algorithm, `i` goes
backward (n-1->1), meaning that the "already" shuffled part of the
array is never altered betweens iterations, while in the current
implementation the "not-yet" shuffled part of the array is
conserved between iterations.

R=golang-dev
CC=golang-dev
https://golang.org/cl/6845121

src/pkg/math/rand/rand.go

@@ -100,7 +100,7 @@ func (r *Rand) Perm(n int) []int {
 	for i := 0; i < n; i++ {
 		m[i] = i
 	}
-	for i := 0; i < n; i++ {
+	for i := 1; i < n; i++ {
 		j := r.Intn(i + 1)
 		m[i], m[j] = m[j], m[i]
 	}