It may be that we have a feedback vector, but no literals. In this case
we can store into the OptimizedCodeMap directly instead of using a WeakCell,
because all data in the feedback vector is already held weakly.

The use of a WeakCell in the OptimizedCodeMap is only required when
there are literals which may hold maps strongly.

This is to address a performance regression caused by the creation of
a large number of WeakCells.

BUG=chromium:615831

Review-Url: https://codereview.chromium.org/2031123003
Cr-Commit-Position: refs/heads/master@{#36786}