Inspired by a remark by Leonard Holz, use constants for division

BenchmarkSprintfEmpty           130           132           +1.54%
BenchmarkSprintfString          438           437           -0.23%
BenchmarkSprintfInt             417           414           -0.72%
BenchmarkSprintfIntInt          663           691           +4.22%
BenchmarkSprintfPrefixedInt     791           774           -2.15%
BenchmarkSprintfFloat           701           686           -2.14%
BenchmarkManyArgs               2584          2469          -4.45%
BenchmarkFprintInt              488           357           -26.84%
BenchmarkFprintIntNoAlloc       402           265           -34.08%
BenchmarkScanInts               1244346       1267574       +1.87%
BenchmarkScanRecursiveInt       1748741       1724138       -1.41%

Update #3463

LGTM=josharian, rsc
R=golang-codereviews, josharian, rsc
CC=golang-codereviews
https://golang.org/cl/144250043