[llvm] [Transforms/Util] Add SimplifySwitchVar pass (PR #149937)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Correct me if I am wrong, but I think we can't do lookup tables efficiently on AMDGPU due to high memory latencies. 

If we can use scalar loads it's probably not that bad, but requires benchmarking 

https://github.com/llvm/llvm-project/pull/149937