The feedback_vector/cell and code fields of a JSFunctionRef are only
used when generating code for the function (e.g., for the function
being optimized or inlined functions). This CL explicitly serializes
these fields only when the function will be used for codegen, otherwise
avoiding their serialization.

BUG=v8:7790,v8:9684

Change-Id: If76bc0b77e51aa10517699e0a9198358fe77f009
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2617083Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#72129}