[llvm] [GuardUtils] Revert llvm::isWidenableBranch change (PR #66411)

[via llvm-commits]

annamthomas wrote:

> For now we still need to check that widenable branch is in the form of: br(widenable_condition & (...))

I think it will be clearer to state why we need to do this check either in the commit message or as a statement in the test added.


https://github.com/llvm/llvm-project/pull/66411