[llvm] [FuncSpec] Update function specialization to handle phi-chains (PR #71442)

[Alexandros Lamprineas via llvm-commits]

labrinea wrote:

Some thoughts. I don't see the point of two lambdas. The reason I originally wrote one is to avoid code repetition. I liked the idea of the lambda returning the constant, but in case of transitive phis it could be that all the incoming values are also transitive phis, which makes it impossible to return a constant. I didn't see the point of keeping three state status for the return value (seen on last revision), a boolean should be sufficient as in the original patch.

https://github.com/llvm/llvm-project/pull/71442