This CL refines https://crrev.com/c/3829539 to only apply to let and
const declared variables. `var`s should stay `undefined`.

R=jarin@chromium.org

Bug: chromium:1328681
Change-Id: I35778c89fb04439348a4f6aebcdeb2db6234f9d0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3848960Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Simon Zünd <szuend@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82640}