math/rand: speed up Float32, Float64

Actually, speed up Int31n and Int63n by avoiding retry loop.

benchmark           old ns/op    new ns/op    delta
BenchmarkFloat32           32           26  -19.45%
BenchmarkFloat64           46           23  -49.47%

Fixes #7267.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/69980047

src/pkg/math/rand/rand.go

@@ -60,6 +60,9 @@ func (r *Rand) Int63n(n int64) int64 {
 	if n <= 0 {
 		panic("invalid argument to Int63n")
 	}
+	if n&(n-1) == 0 { // n is power of two, can mask
+		return r.Int63() & (n - 1)
+	}
 	max := int64((1 << 63) - 1 - (1<<63)%uint64(n))
 	v := r.Int63()
 	for v > max {
@@ -74,6 +77,9 @@ func (r *Rand) Int31n(n int32) int32 {
 	if n <= 0 {
 		panic("invalid argument to Int31n")
 	}
+	if n&(n-1) == 0 { // n is power of two, can mask
+		return r.Int31() & (n - 1)
+	}
 	max := int32((1 << 31) - 1 - (1<<31)%uint32(n))
 	v := r.Int31()
 	for v > max {

src/pkg/math/rand/rand_test.go

@@ -376,6 +376,13 @@ func BenchmarkFloat32(b *testing.B) {
 	}
 }
 
+func BenchmarkFloat64(b *testing.B) {
+	r := New(NewSource(1))
+	for n := b.N; n > 0; n-- {
+		r.Float64()
+	}
+}
+
 func BenchmarkPerm3(b *testing.B) {
 	r := New(NewSource(1))
 	for n := b.N; n > 0; n-- {