[llvm] [SimplifyCFG] When only one case value is missing, replace default with that case (PR #76669)

[via llvm-commits]

alexfh wrote:

>> Could you please revert?
>
> If it does block something important, that's fine.

I've prepared #78469 to revert both commits. Waiting for premerge checks.

https://github.com/llvm/llvm-project/pull/76669