Move the tophash checks after the equality/length checks.

For fast32/fast64, since we've done a full equality check already,
just check whether tophash is empty instead of checking tophash.
This is cheaper and allows us to skip calculating tophash.

These changes are modeled on the changes in CL 57590,
which were polished based on benchmarking.
Benchmarking directly is impeded by #21546.

Change-Id: I0e17163028e34720310d1bf8f95c5ef42d223e00
Reviewed-on: https://go-review.googlesource.com/57611
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>