[llvm] [IR] Accept nonnull metadata on phi nodes (PR #202877)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Last I checked it didn't even try? Should implement that first?

If you mean we should improve `isGuaranteedNotToBeUndef` first, I have a candidate patch for https://github.com/llvm/llvm-project/issues/174813 : https://github.com/llvm/llvm-project/compare/main...dtcxzyw:llvm-project:perf/noundef-rec-v2. 

https://github.com/llvm/llvm-project/pull/202877