Pushing unresolved variables at the front was an optimization for the case
where we didn't have an end pointer. That forces us to do an O(<new elements>)
walk to rescope variables. The implementation was more generic and even did
O(<all elements>). Now that we have an end pointer we can simply push at the
end and MoveTail which is O(1).

Change-Id: I65cd5752b432223d95cd529452a064d8dcc812e1
Reviewed-on: https://chromium-review.googlesource.com/c/1351010
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57868}