r189218 - Simplify now that -O4 just maps to -O3 and -O is an alias of -O2.

[Rafael Espíndola]

> Basically yeah...except I think the vectorizer part was already working since it's parsing the numeric value of the flag.

That would work, but the translation of -O4 to -O3 would be a bit more
complicated without an explicit option for -O4.

I committed the patch as r189369.

> (Originally I thought about specifically warning about -flto, but I suppose that would confuse newcomers.)
>
> Jordan