[llvm] [FastISel][X86] Use getTypeForExtReturn in GetReturnInfo. (PR #80803)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> > Given DAGIsel doesn't do zeroext on callee and both do zeroext in caller https://godbolt.org/z/e4qf3Mdrn. I think we are safe to go with this patch.
> 
> This sounds broken though

The LangRef says it's required by the target’s ABI. I checked 64-bit psABI, and didn't see such requirement. So we don't break anythink here. 32-bit maybe a different story https://godbolt.org/z/be9Pvzecn
Besides, GCC does the same thing here https://godbolt.org/z/9rY4GeGd8

https://github.com/llvm/llvm-project/pull/80803