The immediate itself is just the index, and the local type can easily be
looked up in every environment where the immediate is used. Hence remove
that field.

R=thibaudm@chromium.org

Bug: v8:10576
Change-Id: If3176fa4880a75bdc475ec61dea60e08001220f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2266532
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68544}