Mixing SSE and AVX instructions can cause severe slowdowns, so always
use AVX instructions if available.
The {Movss} method of {TurboAssembler} automatically does this,
emitting either {vmovss} or {movss}.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I45124944964e5a970814c7b50dfbfca333137dc1
Reviewed-on: https://chromium-review.googlesource.com/865910Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50572}