This CL adds support for f64.const, f64.add, f64.sub and f64.mul.

R=ahaas@chromium.org

Bug: v8:6600
Change-Id: I7374ede800db83303c8fa647a183fdda53a151cd
Reviewed-on: https://chromium-review.googlesource.com/913613Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51263}