[llvm] [IR] Add a helper `Function::isReturnNonNull` (PR #128107)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

Interesting: https://alive2.llvm.org/ce/z/4RDBS8
Closed this PR until we need it in the future.


https://github.com/llvm/llvm-project/pull/128107