Two additions are faster than two multiplications and one addition. The
code seems simpler to me too, as it's more obvious that we advance two
destination bytes for each source byte.

name            old time/op  new time/op  delta
Encode/256-8     374ns ± 0%   331ns ± 0%  -11.44%  (p=0.008 n=5+5)
Encode/1024-8   1.47µs ± 0%  1.29µs ± 0%  -11.89%  (p=0.004 n=6+5)
Encode/4096-8   5.85µs ± 1%  5.15µs ± 0%  -11.89%  (p=0.004 n=6+5)
Encode/16384-8  23.3µs ± 0%  20.6µs ± 0%  -11.68%  (p=0.004 n=6+5)

Change-Id: Iabc63616c1d9fded55fa668ff41dd49efeaa2ea4
Reviewed-on: https://go-review.googlesource.com/c/go/+/151198
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: roger peppe <rogpeppe@gmail.com>