The only place in fullcodegen, where we know for sure that a for-in loop
entered the slow-path is right before the potential call to %ForInFilter.
So there's no point in also updating the mode eagerly during ForInPrepare.

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

Review URL: https://codereview.chromium.org/1749033002

Cr-Commit-Position: refs/heads/master@{#34377}