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

[Quentin Dian via llvm-commits]

DianQK wrote:

> I fixed the regression by changing the default case from `%default.unreachable` to another BB :(

An “arbitrarily meaningless” BB?

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