This is a reland of decf5750

This patch fixes the hash code migration in the backing store
transition case from Smi to PropertyArray in the IC system and
Turbofan. Also, adds tests.

Bug: v8:6413, v8:6404

Original change's description:
> [runtime] Store hash code in length field
>
> Store the hash code in 21 bits of the length field.
>
> Change the GetIdentityHash API to be unhandlified, since there's no
> property lookup anymore.
>
> Update js/ and test/ to match new API and expections.
>
> Bug:
> Change-Id: I8dc75de4021f59e79b45f3f38ec997c3b3687b24
> Reviewed-on: https://chromium-review.googlesource.com/589688
> Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#47259}

Change-Id: I69289113c4b7978c46f6f9373cc972086ecb6822
Bug: 
Reviewed-on: https://chromium-review.googlesource.com/614903
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47459}