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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> > Thanks for looking! I'd say if it's not convenient to prepare a fix by around Tuesday, it would likely be better to revert (I can take care of this) until you're back from the holidays. Happy New Year, btw! ;)
> 
> Hi, this is causing quite a significant number of failures in our internal testing - many protocol buffer generated files are timing out. We really appreciate it if you submit the fix soon or revert this until the fix is ready. Thanks!

Potential fix: https://github.com/llvm/llvm-project/pull/78582

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