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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

The current patch shows these [regressions](https://github.com/dtcxzyw/llvm-opt-benchmark/pull/1604):
```
Top 5 regressions:
  llvm/X86EncodingOptimization.cpp.ll 424207707 -> 1488145717 +250.81%
  llvm/TokenKinds.cpp.ll 151977954 -> 475203715 +212.68%
  libquic/net_errors.cc.ll 121648675 -> 366440443 +201.23%
  cvc5/kind.cpp.ll 230067048 -> 560166555 +143.48%
  rust-analyzer-rs/4ifo5x52byu175vr.ll 177309886 -> 407442281 +129.79%
```

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