port 2d281e71 (r27633)

original commit message:

    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.

BUG=

Review URL: https://codereview.chromium.org/1077523002

Cr-Commit-Position: refs/heads/master@{#27693}