runtime: speed up stack copying a little

runtime: speed up stack copying a little

Remove a branch and a stack spill.

name                old time/op  new time/op  delta
StackCopy-8         79.2ms ± 1%  79.1ms ± 2%    ~     (p=0.063 n=96+95)
StackCopyNoCache-8   121ms ± 1%   120ms ± 2%  -0.46%  (p=0.000 n=97+88)

Change-Id: Ifcbbb05d773178fad84cb11a9a6768ace69fcf24
Reviewed-on: https://go-review.googlesource.com/94029
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>