Instead of re-iterating over the heap all the time, use the
list of feedback vectors on the isolate. This also avoids GC of vectors.

Bug: v8:5935
Change-Id: I0bb96fcf2b0feb9856e9806f812188de1fc7b37e
Reviewed-on: https://chromium-review.googlesource.com/668396Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Franziska Hinkelmann <franzih@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48643}