[turbofan] Prepare mechanism to enable TF on language subset.

[turbofan] Prepare mechanism to enable TF on language subset.

This allows enabling TurboFan on a certain subset of language features
in the AstNumberingVisitor. The heuristics of when to optimize remain
unchanged, only the choice of which optimizing compiler to use changes.

R=bmeurer@chromium.org
BUG=v8:4131
LOG=N

Review URL: https://codereview.chromium.org/1155503002

Cr-Commit-Position: refs/heads/master@{#28544}