There is not much missing to support loading and storing f32 values.
This CL adds that.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: If4a21490bfcb9cccec1ba62986a23549b3c857fa
Reviewed-on: https://chromium-review.googlesource.com/866711Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50606}