Seems to be pretty perf-neutral (for the benchmarks we have in the tree) against
the pre-flattened version.

BUG=v8:5388
R=bmeurer@chromium.org, adamk@chromium.org, littledan@chromium.org

Review-Url: https://codereview.chromium.org/2382013002
Cr-Commit-Position: refs/heads/master@{#39907}