[Bug middle-end/90774] avoid doing vector splat arithmetic where possible

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90774

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So this kind of reassociation can only be done with either -fwrapv or unsigned
types.  Due to integer overflow being undefined.

+ is left-to-right associative operator.