This adds a new simd lowering execution mode for
simd and re-enables the lowering tests

R=titzer@chromium.org,gdeepti@chromium.org,bbudge@chromium.org,mtrofin@chromium.org
BUG=v8:6020

Change-Id: Ice6b7ff2f5973804d379c88241d49b811429a965
Reviewed-on: https://chromium-review.googlesource.com/698928
Commit-Queue: Aseem Garg <aseemgarg@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Reviewed-by: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48322}