We don't need to save any data for top-level leaf
funcs (they contain no skippable funcs), so we
don't need scope analysis for them either.

BUG=v8:5516

Change-Id: I75700838a3df2f19da559145611c99e2c7ffd088
Reviewed-on: https://chromium-review.googlesource.com/691976
Commit-Queue: Marja Hölttä <marja@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48746}