Clang doesn't optimise over handle derefs. Change the ValueSerializer
and the JsonStringifier to use InstanceType directly for checks.
This CL squeezes another 1.5% of JSON.stringify in local benchmarks.

Drive-by-fix:
- Avoid a few more derefs in the JsonStringifier
- Make JsonStringifier::SerializeJSArray a bit more readable

Change-Id: I37626a6d92a8d9275611a4e6d1d908f2e0c6d43b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3247637
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77697}