This cuts down the perf cost on Octane from 18% to 13%. The baseline is the no mitigation
Octane score, the array access mitigation cost was about 4%. This means we would be
getting a bit more than 1/3 of the poisoning regression back.

Bug: chromium:856973, chromium:887213
Change-Id: Ibd99f66ae832c6080f2c2e5b33a1a7610907466f
Reviewed-on: https://chromium-review.googlesource.com/c/1251401Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56409}