[llvm] [bpi] Reuse the AsmWriter's BB naming scheme (PR #73593)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse commented:

I'm not an expert on branch-probability facilities, but the test changes look highly valuable,

It seems awkward to add an extra flag to the block-printer, as it introduces a number of conditionals -- would it be possible to refactor as a dedicated  `printBlockName` method rather than an extra option for block printing?

https://github.com/llvm/llvm-project/pull/73593