x86, arm, arm64: no change in behavior
mips, mips64: disasm-mips(64).cc grows an UNREACHABLE that's
              maybe optimistic (but if it's not true, then that
              looks like a current unintentional fallthrough at
              that spot)
test-js-typed-lowering.cc: looks like a clear bug, but test-only code

Follow-up to https://chromium-review.googlesource.com/c/v8/v8/+/911731 which
did this for x64.

Doesn't turn on the warning yet.

Bug: chromium:812686
Change-Id: I7dd79c9885c90f41dd7e3a595256a954ab0ae643
Reviewed-on: https://chromium-review.googlesource.com/923528Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51437}