This significantly speed-ups Trunc.
Ceil/Floor are using the same instruction, so do them too.

name     old time/op  new time/op  delta
Floor-6  3.33ns ± 1%  3.22ns ± 0%   -3.39%  (p=0.000 n=10+10)
Ceil-6   3.33ns ± 1%  3.22ns ± 0%   -3.16%  (p=0.000 n=10+7)
Trunc-6  4.83ns ± 0%  3.22ns ± 0%  -33.36%  (p=0.000 n=6+8)

Change-Id: If848790e458eedfe38a6a0407bb4f589c68ac254
Reviewed-on: https://go-review.googlesource.com/68630
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>