This is a reland of 5d7a29c9

This reland shuffles around the order of checks in Heap::AllocateRawWith
to not check the new space addresses until it's known that this is a new
space allocation. This fixes an UBSan failure during read-only space
deserialization, which happens before the new space is initialized.

It also fixes some issues discovered by --stress-snapshot, around
serializing ThinStrings (which are now elided as part of serialization),
handle counts (I bumped the maximum handle count in that check), and
clearing map transitions (the map backpointer field needed a Smi
uninitialized value check).

Original change's description:
> [serializer] Allocate during deserialization
>
> This patch removes the concept of reservations and a specialized
> deserializer allocator, and instead makes the deserializer allocate
> directly with the Heap's Allocate method.
>
> The major consequence of this is that the GC can now run during
> deserialization, which means that:
>
>   a) Deserialized objects are visible to the GC, and
>   b) Objects that the deserializer/deserialized objects point to can
>      move.
>
> Point a) is mostly not a problem due to previous work in making
> deserialized objects "GC valid", i.e. making sure that they have a valid
> size before any subsequent allocation/safepoint. We now additionally
> have to initialize the allocated space with a valid tagged value -- this
> is a magic Smi value to keep "uninitialized" checks simple.
>
> Point b) is solved by Handlifying the deserializer. This involves
> changing any vectors of objects into vectors of Handles, and any object
> keyed map into an IdentityMap (we can't use Handles as keys because
> the object's address is no longer a stable hash).
>
> Back-references can no longer be direct chunk offsets, so instead the
> deserializer stores a Handle to each deserialized object, and the
> backreference is an index into this handle array. This encoding could
> be optimized in the future with e.g. a second pass over the serialized
> array which emits a different bytecode for objects that are and aren't
> back-referenced.
>
> Additionally, the slot-walk over objects to initialize them can no
> longer use absolute slot offsets, as again an object may move and its
> slot address would become invalid. Now, slots are walked as relative
> offsets to a Handle to the object, or as absolute slots for the case of
> root pointers. A concept of "slot accessor" is introduced to share the
> code between these two modes, and writing the slot (including write
> barriers) is abstracted into this accessor.
>
> Finally, the Code body walk is modified to deserialize all objects
> referred to by RelocInfos before doing the RelocInfo walk itself. This
> is because RelocInfoIterator uses raw pointers, so we cannot allocate
> during a RelocInfo walk.
>
> As a drive-by, the VariableRawData bytecode is tweaked to use tagged
> size rather than byte size -- the size is expected to be tagged-aligned
> anyway, so now we get an extra few bits in the size encoding.
>
> Bug: chromium:1075999
> Change-Id: I672c42f553f2669888cc5e35d692c1b8ece1845e
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2404451
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Jakob Gruber <jgruber@chromium.org>
> Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#70229}

Bug: chromium:1075999
Change-Id: Ibc77cc48b3440b4a28b09746cfc47e50c340ce54
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2440828
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70267}