[llvm] [DominanceFrontier] make iterating dereferenced DominanceFrontierBase::find deterministic (PR #69711)

[Wenju He via llvm-commits]

wenju-he wrote:

> Would be nice to have a test that expects a specific order regardless of pointer values.

Done. Updated the test. 
In the original test `new_pm_test.ll`, the print order is always deterministic w/ or w/o this PR.
I added `a` prefix to a few basic block labels so that the test will fail w/o this PR.

https://github.com/llvm/llvm-project/pull/69711