[llvm] [llvm] Remove `br i1 undef` from regression tests (PR #121733)

[Lee Wei via llvm-commits]

leewei05 wrote:

I think it is due to the name of the test function. 
```
define void @undef_2phi ...
define void @foo_undef ...
```

One way is to change the deprecator's regex, another solution is to change the function name to `poison_2phi`, `foo_poison`. @nunoplopes What do you think?


https://github.com/llvm/llvm-project/pull/121733