This CL replaces 'let' with 'const' where applicable. This will
generate TNodes instead of TVARIABLEs in the resulting CSA code.

R=jgruber@chromium.org

Bug: v8:8015
Change-Id: I806702c1bfa141e4c934a83c34dd49c321e18ce7
Reviewed-on: https://chromium-review.googlesource.com/1169811Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Simon Zünd <szuend@google.com>
Cr-Commit-Position: refs/heads/master@{#55033}