- Avoid handle derefs where possible
- Split off PostProcessNewJSReceiver to avoid additional instance-type
  checks
- Precompute should_rehash_ to avoid additional branches in
  PostProcessNewObject

Bug: v8:12195
Change-Id: Ib80e711ced48b9b43072ada4e7ed72eb11ab0b8c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3270537Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79279}