[es6] Fix completion values of for loops with lexical variables

[es6] Fix completion values of for loops with lexical variables

Currently, the desugaring of for loops of the form for
(let/const ...; bla; bla) causes them to always have a
completion value of 1, regardless of whether the loop body
is executed or not. This CL fixes this, realigning
initializer blocks as a more general purpose way to avoid
the completion value rewriter (since that's all they really
do anyway).

BUG=

Review URL: https://codereview.chromium.org/1177053006

Cr-Commit-Position: refs/heads/master@{#29108}