Make String::IsEqualTo use a direct string shape dispatch and a direct
call to CompareChars, rather than splitting the behaviour over
IsOneByte/IsTwoByte/HasOneBytePrefix. Avoiding GetFlatContent will make
this method easier to make efficient while staying string-access-lock
safe.

Also, redefines the sequential string table key's matcher in terms of
this IsEqualTo method.

Change-Id: Iab71246e12044ebaeff06f0dbc14d28b3482dcbe
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2557979
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71378}