Have clear prologue and epilogue steps. For a minor MC we can then
templatize the prologue without changing the actual evacuation routine.

BUG=chromium:651354

Change-Id: Ia238748e90bc6b616cd813d31198de182fe11498
Reviewed-on: https://chromium-review.googlesource.com/445898Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43378}