[llvm] [IR] Add new function attribute nocreateundeforpoison (PR #164809)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Does NoCreateUndefOrPoison imply propagating poison?

No. Just like ValueTracking's canCreateUndefOrPoison, it tells you whether the result might be undef/poison when the arguments are not undef/poison. It says nothing about the result when the arguments _are_ undef/poison. That would need a separate attribute.

https://github.com/llvm/llvm-project/pull/164809