Utf8Decoder used to use unibrow::Utf8::ValueOfIncremental, which had a
fast path to avoid the decoder for bytes less than 0x80 in the start
state.  We had to switch away from ValueOfIncremental but it's probably
a good idea to keep the fast path.

Bug: v8:12868
Change-Id: I7d83d67f2c13a1c4f026dde04ef0a69b7de47dc3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3723498
Commit-Queue: Andy Wingo <wingo@igalia.com>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81377}