For some reason, the DisableCrankshaft() in ast-numbering.cc does not always
prevent crankshaft from happening. Bailout here rather than asserting an
unreachable condition.

BUG=546967, v8:4488
LOG=N
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1414713004

Cr-Commit-Position: refs/heads/master@{#31537}