Port 2d281e71

Original commit message:
This enables eager optimization of top-level code with TurboFan and
extends test coverage by triggering it with the --always-opt flag.
Script contexts are now also properly allocated in TurboFan.

BUG=

Review URL: https://codereview.chromium.org/1066843005

Cr-Commit-Position: refs/heads/master@{#27643}