In case the receiver map has an enum cache, %ForInPrepare returns the
length of the actual enum cache, which might include properties that
are further down the transition tree tho.

R=jarin@chromium.org
BUG=v8:3650
LOG=n

Review URL: https://codereview.chromium.org/1619353002

Cr-Commit-Position: refs/heads/master@{#33469}