[Bug middle-end/82853] Optimize x % 3 == 0 without modulo

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82853

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
Might be even more special-cased with x % 3 == 0 ? C1 : C2 aka in if-conversion
context.