[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?
> 
> Yeah :)

I have considered a similar approach in my optimization PR. I'm pretty sure this has to be the wrong approach. Since unreachable provides more facts, this will definitely help with the final machine code. Also see https://github.com/llvm/llvm-project/pull/78582#issuecomment-1898566450.

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