Use them to encode int32 constants properly.
This reduces the generated wasm size in the unity benchmark from
21.6 MB to 16.8 MB (-22.2%).
This hopefully also translates to increased performance especially on
mobile because of lower memory usage.

R=bradnelson@chromium.org, titzer@chromium.org

Review-Url: https://codereview.chromium.org/2692943002
Cr-Commit-Position: refs/heads/master@{#43200}