Ideally, in the common case the backtracking stack should be
stack-allocated (and thus cheap). We should only switch to dynamic
allocation if needed. SmallVector implements exactly this strategy, so
switch to that as a backing store.

This improves Octane/RegExp scores (--regexp-interpret-all) by 50%.

Bug: v8:7777,v8:9330
Change-Id: I0d1b07bd8fd94483128e021390d054f483076f8d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1645318
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62013}