Change the return thunk to use lahf/shaf.
Change the return thunk to use lahf/shaf.

This yields another ~10% improvement to the profiler micro-benchmark.

R=sebmarchand@chromium.org

Review URL: https://codereview.appspot.com/7593044

git-svn-id: db59699583a60be9a535cd09cdc9132301867226@1385 15e8cca8-e42c-11de-a347-f34a4f72eb7d

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