Otherwise, the quiet NaN bit might flip already when loading the
float/double from memory or storing it.
This fixes another NaN bit flip which happened on a single bot only.

R=titzer@chromium.org

Bug: v8:6954
Change-Id: Ica9be71db9c5b505302686e9c0a4b1cae020a7e4
Reviewed-on: https://chromium-review.googlesource.com/735320Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48900}