Add support for f32.demote/f64 and f64.promote/f32.
Both are straight forward to implement on ia32 and x64.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I75213ddf5f422d0aa1d5d9a3bbc90ac32f6950f5
Reviewed-on: https://chromium-review.googlesource.com/955849
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51872}