With these accessors we can remove Assembler as a friend class.

Drive-by cleanup to change DCHECK(!x || y) to DCHECK_IMPLIES(x, y).

Change-Id: I74b7a23e85b50db93bbfe84fdfcc8563527f14d2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3144374Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Zhi An Ng <zhin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76757}