This change partially decouples the heap's allocation space numbering
from the snapshot space encoding. This allows encoding read-only
heap allocations when RO_SPACE is removed.

A few pieces of src/snapshot also get cleaner as they no longer need to
check for extraneous AllocationSpace values.

Bug: v8:7464
Change-Id: I984c039b0e50e233209911ac3e655eb39be2551b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1690956
Commit-Queue: Maciej Goszczycki <goszczycki@google.com>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62574}