runtime: speed up growslice by avoiding divisions 2

runtime: speed up growslice by avoiding divisions 2

This is a follow-up of https://go-review.googlesource.com/#/c/20653/

Special case computation for slices with elements of byte size or
pointer size.

name                      old time/op  new time/op  delta
GrowSliceBytes-4          86.2ns ± 3%  75.4ns ± 2%  -12.50%  (p=0.000 n=20+20)
GrowSliceInts-4            161ns ± 3%   136ns ± 3%  -15.59%  (p=0.000 n=19+19)
GrowSlicePtr-4             239ns ± 2%   233ns ± 2%   -2.52%  (p=0.000 n=20+20)
GrowSliceStruct24Bytes-4   258ns ± 3%   256ns ± 3%     ~     (p=0.134 n=20+20)

Change-Id: Ice5fa648058fe9d7fa89dee97ca359966f671128
Reviewed-on: https://go-review.googlesource.com/21101
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>