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

[Jay Foad via llvm-commits]

jayfoad wrote:

FYI this has made a huge improvement in the time it takes `clang -O3` to compile `lib/Target/AMDGPU/AsmParser/AMDGPUAsmParser.cpp` -- more than 2x improvement on trunk, and more than 10x improvement in a downstream branch where the generated AsmParser is larger. So thank you!

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