[compiler] Various heap broker cleanups

- Simplify some macros.
- Simplify some handle creations.
- Make various accessors more uniform.
- Remove leftover assumptions about serialized children.

Change-Id: Iee2951065c442aba1b479a48de33f0b8e0c7b057
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2402033
Commit-Queue: Georg Neis <neis@chromium.org>
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Santiago Aboy Solanes <solanes@chromium.org>
Auto-Submit: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69807}