This is an alternative way of D139627 suggested by Craig. Creently only X86 backend uses this attribute. Let's just emit for X86 only.
It's an attractive suggestion. But it's not easy to move it into target code because the value is not a static one but calculated during codegen. As I explained in LangRef, the value is max(maxVectorWidthInArgsRet(CurFn), maxVectorWidthInArgsRet(Callee0), maxVectorWidthInArgsRet(Callee1), ...). We may need to copy/paste a lot of codegen code there to achieve the same goal.