Previously we created a duplicate map because we couldn't set the
correct ElementsKind when we created the function. This patch updates
the ElementsKind field of the existing map directly instead of
creating a new one.

This patch also correctly sets the instance_size when we create the
map the first time.

Review-Url: https://codereview.chromium.org/2486193003
Cr-Commit-Position: refs/heads/master@{#40872}