Incooperate suggestion from adamk@ to only sent lexical variables to
I+TF that require explicit initialization, i.e. don't send named
function expressions to I+TF. This should recover most of the regression
now.

Also introduce a regression test for the original let issue.

BUG=chromium:670691,v8:5666
R=adamk@chromium.org,yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2556663002
Cr-Commit-Position: refs/heads/master@{#41507}