By reusing a single vector instead of allocating one per code object,
we can save lots of heap allocations.

R=eholk@chromium.org

Bug: v8:5277
Change-Id: Ia462c97293cd00607d9f2faf29e265ea78d49394
Reviewed-on: https://chromium-review.googlesource.com/686819Reviewed-by: Eric Holk <eholk@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48187}