We checked against kNotFound twice; once in
TransitionsAccessor::SearchTransitions and once in TransitionArray::Search. It's
unnecessary to check twice.

In local tests, this speeds up searching for a non-existent transition in a
2-element TransitionArray by ~20%.

BUG=v8:8547

Change-Id: Ic220c4c0bfeeacd9b81953efad212ed15168437e
Reviewed-on: https://chromium-review.googlesource.com/c/1361167Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58038}