[llvm] [SimplifyCFG] Simplify switch instruction that has duplicate arms (PR #114262)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I am surprised to see the compile time regress in https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1597 compared to https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1595. 

I would be surprised if maintaining the `Seen` set is worse than building `PhiPredIVs` an extra time. Is there something that I am missing?

https://github.com/llvm/llvm-project/pull/114262