[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.

It may still be worth doing this to avoid spurious invalidation of CFG analyses, but it's not really related to this patch...

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