Varblock scopes can be treated as the "same scope" as their surrounding
function scope for the purposes of hole check elimination, as
source position comparison is sufficient to determine statically that
uses in the varblock scope are after initialization in the function scope.

This allows the elimination of hole checks of lexically-bound parameter
variables in functions with complex parameters, including rest parameters.

The pre-existing code compared DeclarationScopes for legacy reasons:
varblock scopes (and Scope::GetClosureScope()) did not exist at the
time this code was originally written.

R=neis@chromium.org

Bug: v8:6344, v8:6414
Change-Id: Ie787d58d1ea172e893788a9c716d3b6868980ab8
Reviewed-on: https://chromium-review.googlesource.com/508242
Commit-Queue: Adam Klein <adamk@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45405}