This CL ensures that elements kind transitions don't cause silent
mutable-to-constant or any-to-class-type migrations of in-place
generalizable fields.

Bug: v8:5495, chromium:783132
Change-Id: Ie60224db62bd45d27148ae0469c7af5a3fe944fd
Reviewed-on: https://chromium-review.googlesource.com/785190Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49583}