[llvm] [SimplifyCFG] Improve FoldTwoEntryPHINode when one of phi values is undef (PR #69021)

[Nikita Popov via llvm-commits]

nikic wrote:

So this now works via programUndefinedIfPoison, which is going to make this quite fragile. It's going to work if this is the first branch in the function, but not in a later one.

Also, while https://github.com/llvm/llvm-project/issues/67342 reported this without the `noundef` attribute, the original in https://github.com/rust-lang/rust/issues/116150 does have it, so I don't think you're even fixing the right issue here.

https://github.com/llvm/llvm-project/pull/69021