Make --always-opt also optimize top-level code.

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.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1053063003

Cr-Commit-Position: refs/heads/master@{#27633}