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

[Jessica Del via llvm-commits]

OutOfCache wrote:

I opened this PR to first refactor SimplifyCFG: https://github.com/llvm/llvm-project/pull/155602 A later PR will do the actual check for target support for LUT.

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