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

In the d6e7c162 was introduced util to
to extract widenable conditions from branch. That util was applied in
the llvm::isWidenableBranch to check if branch is widenable. So we
consider branch is widenable if it has widenable condition anywhere in
the condition tree. But that will be true when we finish GuardWidening
reworking from branch widening to widenable conditions widening.
For now we still need to check that widenable branch is in the form of:
`br(widenable_condition & (...))`,
because that form is assumed by LoopPredication and GuardWidening
algorithms.

Fixes: https://github.com/llvm/llvm-project/issues/66418



Co-authored-by: Aleksander Popov <apopov@azul.com>