[Bug c/80852] Optimisation missed for loop with condition that is always true

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80852

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
I don't think the test on x%2 is relevant, gcc eliminates it without problem. I
thought gcc had some code to recognize 1+2+3+...+n and similar patterns, but
apparently not this one.