This might or might not give clang-tidy a hint that the reported case
(see issue) cannot happen. It might also generate slightly better code
by giving hints to the compiler.
Note that V8_ASSUME is actually a DCHECK in DEBUG builds, so we do not
loose any checks here.

Some DCHECKs were removed because they are redundant
(RegisterBase::code() assumes to be only called on valid registers).

R=jkummerow@chromium.org

Bug: chromium:1349619
Change-Id: I467e7917a87ec86dd692f0edeed6bb72e0393cc4
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3804667
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82273}