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

[via llvm-commits]

DianQK wrote:

Hmm, I think we need to address the regression issue I mentioned earlier. I believe that adding the `SimplifySwitchWithImplicitDefault` option is always suboptimal, especially for unrelated to the target.

I think we can first transform the default branch to unreachable. And then make the transform in another PR when we find it in consuming this UB. I believe this is what makes a UB valuable.

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