[llvm] [SimplifyCFG] Simplify switch with implicit default (PR #95665)

[via llvm-commits]

DianQK wrote:

Could you put your code in `eliminateDeadSwitchCases`?  As for how to handle unreachable cases, I think we can introduce a new transformation (or `simplifyUnreachable`).

https://github.com/llvm/llvm-project/pull/95665