This is a reland of 29585a06 after
removing an incorrect DCHECK.

Original change's description:
> [turbofan] Various serializer/broker improvements
>
> They are all somewhat entangled, sorry for the big CL.
>
> - Brokerize remaining feedback vector slots.
> - Introduce Hints::SingleConstant helper.
> - Introduce SerializationPolicy enum.
> - Eliminate use of nullptr for megamorphic load/store ic feedback.
>   Instead use the corresponding ProcessedFeedback with an empty list
>   of maps or the like. new class MegamorphicFeedback.
> - Separate processing of feedback from serialization. This eliminates
>   code duplication.
> - Be very careful when clearing hints not to overwrite hints that are
>   being processed.
> - Move AccessInfos out of NamedAccessFeedback. Always store them in
>   property_access_infos_ map on broker. (This was actually unused
>   before, somewhat by mistake.)
> - Support map inference in concurrent inlining. Rewrite
>   ElementAccessFeedback such that we can refine it with the set of
>   inferred maps.
>
> TBR: mvstanton@chromium.org
> Change-Id: I05e9eb250bdffc6dff29db01742550a86a41cb31
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1752853
> Commit-Queue: Georg Neis <neis@chromium.org>
> Reviewed-by: Georg Neis <neis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#63232}

TBR: mvstanton@chromium.org
Bug: v8:7790
Change-Id: Ia4acd31b339a941ee065e1ae4835bb7b85d5685e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1758319Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63241}