... for visiting slots containing pointers to Code objects when
external code space mode is enabled.
These slots will require different handling once the code space is
moved out of the V8 heap cage.

This CL also introduces IsValidCodeObject() predicate similar to
IsValidHeapObject() for checking if given HeapObject is a valid Code
object.

Tbr: cbruni@chromium.org
Bug: v8:11880
Change-Id: I430940f4503cebfd2a6d387e44349810991a93e9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3032085Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75787}