[llvm] [PredicateInfo] Support existing `PredicateType` by adding `PredicatePHI` when needing introduction of phi nodes (PR #151132)

[Nikita Popov via llvm-commits]

nikic wrote:

Can you please add a test that shows the generated IR? It's hard to comment in abstract.

https://github.com/llvm/llvm-project/pull/151132