[llvm] [SCCP] Check whether the default case is reachable (PR #76295)

[Nikita Popov via llvm-commits]

nikic wrote:

> What happens if the default is already unreachable? Will we replace it with a new unreachable block? It would be good to skip the update in that case.

Hm, I guess this isn't so simple because we'll mark the block as dead and remove it.

https://github.com/llvm/llvm-project/pull/76295