In IdentityMap, explicitly pass the key's hash so that it can be cached
between Lookup and Insert.

Change-Id: Ib8a2d96cc399ae025f54c61c129dd4cd18d86c7e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2448795
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70319}