runtime: remove indentation in evacuate

runtime: remove indentation in evacuate

Combine conditions into a single if statement.
This is more readable.

It should generate identical machine code, but it doesn't.
The new code is shorter.

Change-Id: I9bf52f8f288b0df97a2b9b4e4183f6ca74175e8a
Reviewed-on: https://go-review.googlesource.com/54651
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>