[Bug c/66348] Simple loop has only lower half of the 64-bit counter initialized correctly

[sebastiano.vigna at unimi dot it]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66348

--- Comment #6 from Sebastiano Vigna <sebastiano.vigna at unimi dot it> ---
I forgot an important aspect: with -fsanitize=undefined the optimization bug
does not show up. The instrumentation perturbs the code enough to make it go
away.