[llvm] Remove `br i1 undef` from regression tests (PR #115130)

[John Regehr via llvm-commits]

regehr wrote:

thanks for doing this!
besides what other people said, I recommend starting small-- like maybe change no more than 20 tests at a time until you get more experience doing this. the purpose of this is to increase the odds that all 20 (or whatever) of your changes are good ones, and to avoid fatiguing your code reviewers.

https://github.com/llvm/llvm-project/pull/115130