[Bug c/91312] -Wconversion warning with += operator

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91312

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Kostas Sotiropoulos from comment #4)
> Any comments on my questions?

Yes go read the c standard about prompting to int here.