Apply optimizations used to speed up YCbCrToRGB from
https://go-review.googlesource.com/#/c/21910/
to RGBToYCbCr.

name             old time/op  new time/op  delta
RGBToYCbCr/0-2   6.81ns ± 0%  5.96ns ± 0%  -12.48%  (p=0.000 n=38+50)
RGBToYCbCr/Cb-2  7.68ns ± 0%  6.13ns ± 0%  -20.21%  (p=0.000 n=50+33)
RGBToYCbCr/Cr-2  6.84ns ± 0%  6.04ns ± 0%  -11.70%  (p=0.000 n=39+42)

Updates #15260

Change-Id: If3ea5393ae371a955ddf18ab226aae20b48f9692
Reviewed-on: https://go-review.googlesource.com/22411Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ralph Corderoy <ralph@inputplus.co.uk>