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