This requires putting the original loop's body inside an inner for loop (with
the same labels as the original loop) and re-binding the temp variables in its
"next" expression. A second flag is added to the desugared code to ensure the
loop body executes at most once per loop.

BUG=v8:3683
LOG=y

Review URL: https://codereview.chromium.org/720863002

Cr-Commit-Position: refs/heads/master@{#25363}