Refactors weak prefix handling, in particular the post-hoc weak prefix
read and HeapObjectReference creation, to a few function calls. This
simplifies ReadDataCase sufficiently that it can be inlined into
ReadData, which removes the need for a) having two places where we
branch on the bytecode value (ReadData and ReadDataCase), and b)
removes the need for the macro helper which calls ReadData. With a
bit of refactoring we can therefore make the big switch much more
explicit.

This patch also moves that switch into a per-bytecode helper, so that
switch entries can return the updated slot, rather than remembering to
update in-place and continue looping.

It also moves the weak prefix handling from the deserializer allocator
to the deserializer itself, as weak prefixes don't have anything to do
with allocation.

Change-Id: I84fbda021cb65d5bfb91fc3ef27f72823acee05a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2395557
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#69800}