Move the check near the code in evacuate that relies on
the relation evacuateX+1 == evacuateY.

If the relation is fullfilled the check is known to be true
at compile time and removed by the compiler.

Change-Id: I711b75e09047bf347819ccaeec41d244a5883867
Reviewed-on: https://go-review.googlesource.com/62410
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>