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

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> > I fixed the regression by changing the default case from `%default.unreachable` to another BB :(
> 
> An “arbitrarily meaningless” BB?

Yeah :)


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