[llvm] [SimplifyCFG] Simplify switch default branch when branch proves operand value (PR #206597)

[Sayan Sivakumaran via llvm-commits]

sivakusayan wrote:

The windows linker in CI seems to be flaky: https://github.com/llvm/llvm-project/issues/177477

Anyway, sorry for the previous mess, this should be ready to review again

https://github.com/llvm/llvm-project/pull/206597