This tweak was already present in CrankShaft for the non-AVX case. As it
turns out, it's also relevant even with AVX. Now the same optimization
is applied in case of TurboFan as well.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/1081033003

Cr-Commit-Position: refs/heads/master@{#27774}