This is a reland of 6f93d59d.
One more test had to be disabled (tracked by bug 6954), and
two machops tests needed to be changed to use boxed floats
and doubles.

Original change's description:
> [test] Add nan bit patterns to uint{32,64}_vector
> 
> If you just cast those patterns to float or double and pass them
> around, the quiet/signaling NaN bit might change. We had several bugs
> around this, so add these patterns to the general input vectors.
> 
> This uncovers a bug in the wasm interpreter, which will be fixed in a
> separate CL.
> 
> R=ahaas@chromium.org
> 
> Bug: v8:6947, v8:6954
> Change-Id: I205b8ab784b087b1e4988190fa725df0b90e7ee0
> Reviewed-on: https://chromium-review.googlesource.com/725345
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Reviewed-by: Andreas Haas <ahaas@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#48731}

Bug: v8:6947, v8:6954
Change-Id: I9a38b5d9324131c3950c537910371a73c93d2c13
Reviewed-on: https://chromium-review.googlesource.com/728439Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48780}