Remove almost all remaining CHECK(false)s

In most cases NOTREACHED() is now a better option. Also performs
dead-code removal.

Some remaining ones can't easily be [[noreturn]] (upstream code doesn't
build with -Wunreachable-code), so we'll need to find another option
when CHECK(false) is understood as [[noreturn]].

Bug: 40122554
Change-Id: I0de1d4fe3567dc220f5c29080b786a1a8fb6b4f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6038634
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Owners-Override: Lei Zhang <thestig@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1388478}