No functional changes; tophash is inlined.

Change-Id: Ic8ce95b3622eafbddcfbc97f8c630ab8c5bfe7ad
Reviewed-on: https://go-review.googlesource.com/55233
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>