The result of an f64 binop was marked as f32 on Liftoffs value stack.
This lead to errors and is fixed in this CL.
I plan to clean up all binop implementions in a follow-up CL.

R=titzer@chromium.org

Bug: chromium:812005, v8:6600
Change-Id: I5bcd5c2e7d2b6170ef60f5e83cf2876b3475c38a
Reviewed-on: https://chromium-review.googlesource.com/924025Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51375}