[llvm] [CVP] Check whether the default case is reachable (PR #79993)

[via llvm-commits]

alexfh wrote:

The original code is coming from a different file from what @dwblaikie used to prepare the reduced example in https://github.com/llvm/llvm-project/pull/76669#issuecomment-1894679825.

https://github.com/llvm/llvm-project/pull/79993