let/const declarations in "standard" C-style for-loops have
some complex desugaring to accommodate the case where loop
loop variables may be captured. This slows down the baseline
performance of for-loops with let variables.

This change attempts to avoid this desugaring if it's known that
the loop variable is not captured at any point. A side effect of
this change is that let/const loop variables, when not captured
within the loop body, are not necessarily shown in the debugger,
similar to other stack-allocated vars.

BUG=v8:4762, v8:5460
R=marja@chromium.org, adamk@chromium.org, yangguo@chromium.org

Change-Id: I8dbe545a12c086f675972bdba60c94998268311a
Reviewed-on: https://chromium-review.googlesource.com/472247
Commit-Queue: Caitlin Potter <caitp@igalia.com>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44731}