Also do very minor code cleanup.

name                old time/op  new time/op  delta
StackCopyPtr-8      84.8ms ± 6%  82.9ms ± 5%  -2.19%  (p=0.000 n=95+94)
StackCopy-8         68.4ms ± 5%  65.3ms ± 4%  -4.54%  (p=0.000 n=99+99)
StackCopyNoCache-8   107ms ± 2%   105ms ± 2%  -2.13%  (p=0.000 n=91+95)

Change-Id: I2d85ede48bffada9584d437a08a82212c0da6d00
Reviewed-on: https://go-review.googlesource.com/109001
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>