This is pre-work for a refactoring that changes how heap objects
are handled in the assembler: Currently, we put the handle location in
the constant pool, and replace these with the actual heap object when
we copy the code from the assembler's buffer to the heap.

In the future, we will put a small index in the constant pool, which
will ultimately enable 32bit constant pool slots for compressed heap
objects. This small index will be fixed up when we copy the code to
the heap.

This CL makes the assembler tests copy the code to the heap, which
ensures that the fix-up phase is actually run.

Change-Id: I80cd69dc57414a3bd0a27f8d558616aadcae05a2
Bug: v8:7703
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1647166
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62112}