[llvm] [SimplifyCFG] Don't use a mask for lookup tables generated from switches with an unreachable default case (PR #94468)

[via llvm-commits]

DianQK wrote:

> [#94468 (comment)](https://github.com/llvm/llvm-project/pull/94468#discussion_r1630381209) I like `AllHolesAreUndefined`, have changed it to that.
> 
> [#94468 (comment)](https://github.com/llvm/llvm-project/pull/94468#discussion_r1630383915) Yeah, I'm definitely interested in doing that, I'll get working on that as soon as this is merged. @DianQK

Thanks. I prefer `AllHolesArePoison` because unreachable immediately results in undefined behavior, so we can use `poison`, which is stronger than `undef`. We can discuss it in the next PR. :)

https://github.com/llvm/llvm-project/pull/94468