Reuse helpers and move generic functions to char-predicates

Change-Id: I63bf704bdd8e8cd788e80ad2d42b689527865e00
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1653118Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62116}