Rather than having repeated IsFoo checks in PostProcessNewObject, which
means repeated handle accesses, map word accesses, and map pointer
decompressions, cache the instance type once and check it with
InstanceTypeChecker.

This gives a measurable 2-3% improvement in deserialization time (in my
informal local measurements).

Bug: chromium:1075999
Change-Id: I3e11588ad5d1c6ee2bbf93b82fa52c66496a325c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440578
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70301}