[Bug c/80253] Optimization silences __attribute__((fallthrough)) warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80253

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Looks like expand_FALLTHROUGH_r should be using case_label_p.