With this, `Builtins::IsBuiltinId(code->builtin_index())` turns into
`Builtins::IsBuiltin(code)`.

Bug: v8:6666
Change-Id: Id731299cf5eb3f213933d3a9b3ae78d9bb95e757
Reviewed-on: https://chromium-review.googlesource.com/937205
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51562}