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

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

I am getting the following results in my local machine:

```
Compilation time result (by files):
Top 5 improvements:
  lightgbm/c_api.cpp.ll 16647754205 -> 16139261311 -3.05%
  faiss/utils.cpp.ll 1544838129 -> 1498931464 -2.97%
  faiss/IndexPQ.cpp.ll 4512055830 -> 4408179250 -2.30%
  gromacs/pme_redistribute.cpp.ll 1325856070 -> 1295687698 -2.28%
  gromacs/kerneldispatch.cpp.ll 614810494 -> 603676278 -1.81%
Top 5 regressions:
  gromacs/mdatoms.cpp.ll 1883643129 -> 1927946180 +2.35%
  lightgbm/metric.cpp.ll 8668505101 -> 8848219689 +2.07%
  lightgbm/linear_tree_learner.cpp.ll 11831124326 -> 12066920031 +1.99%
  meshlab/arap.cpp.ll 20066421231 -> 20418357293 +1.75%
  faiss/Index2Layer.cpp.ll 479186376 -> 487159874 +1.66%

Overall: 0.00054018%
```

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