... a Smi-looking type containing properly sign-extended int31 integer.

The idea is to use this kind of tagged integers for the cases where
the value is guaranteed to fit into int31. For example, feedback vector
slots is one of the candidates for using TaggedIndex representation.

Bug: v8:10047
Change-Id: Ifaa2978a5d42467578ff243dc44d327536efbe93
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1960292Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66170}