This fixes corner cases where the layout of feedback vectors baked into
the snapshot is different from the expected layout, depending on some
runtime flags. We make sure the feedback vector is regenereated for
functions that are not compiled. Flag changes of this kind are only
allowed when code is not serialized.

An alternative solution would be to not serialize the feedback vector
for such cases in the first place. That solution however would have a
higher overhead, as it would required the serializer to be able to
recognize feedback vectors while generating a snapshot.

R=mvstanton@chromium.org
TEST=mjsunit/regress/regress-crbug-600995
BUG=chromium:600995
LOG=n

Review URL: https://codereview.chromium.org/1869693003

Cr-Commit-Position: refs/heads/master@{#35339}