[llvm] Add EliminateNewDuplicatePHINodes function. (PR #135179)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

I don't like how much code this is duplicating. This will make it hard to improve EliminateDuplicatePHINodes in the future.

I suggested to use this helper because I thought it was going to be simpler, but if this just adds more complexity in the end, I'm totally fine with a simple implementation in SSAUpdaterBulk.

https://github.com/llvm/llvm-project/pull/135179