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

[Nikita Popov via llvm-commits]

nikic wrote:

I thought it might be beneficial to integrate a block -> index cache directly in PHINode, but at least a naive attempt did not yield good results: https://llvm-compile-time-tracker.com/compare.php?from=e52238b59f250aef5dc0925866d0308305a19dbf&to=bf327b327f172d807b6caa07378e85ede7e83956&stat=instructions:u

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