This implements f64 comparisons analogue to f32 comparisons (f64.eq,
f64.ne, f64.lt, f64.gt, f64.le, f64.ge).

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I382d575e4f2c585c2de1313f2c2fd2c8cda54a9d
Reviewed-on: https://chromium-review.googlesource.com/995773
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52390}