For the purpose of graph verification, a SandboxedPointer is equivalent
to a Word64. The SandboxedPointer representation only affects lowering,
which needs to perform encoding/decoding of the pointer when
storing/loading it to/from the heap. As such, this CL promotes
kSandboxedPointer to kWord64 representation for graph verification.

Bug: v8:10391
Change-Id: I466f03dc7bbe9ed8f7b4f431aee5b14bd6db3d31
Cq-Include-Trybots: luci.v8.try:v8_linux64_heap_sandbox_dbg_ng,v8_linux_arm64_sim_heap_sandbox_dbg_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3483658Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79225}