[BPI] Reuse the AsmWriter's BB naming scheme in BranchProbabilityPrinterPass (#73593)

When using `BranchProbabilityPrinterPass`, if a BB has no name, we get pretty unusable information like `edge -> has probability...` (i.e. we have no idea what the vertices of that edge are).

This patch uses `printAsOperand`, which uses the same naming scheme as `Function::dump`, so for example during debugging sessions, the IR obtained from a function and the names used by `BranchProbabilityPrinterPass` will match.

A shortcoming is that `printAsOperand` will result in the numbering algorithm re-running for every edge and every vertex (when `BranchProbabilityPrinterPass` is run on a function). If, for the given scenario, this is a problem, we can revisit this subsequently.

Another nuance is that the entry basic block will be numbered, which may be slightly confusing when it's anonymous, but it's easily identifiable - the first edge would have it as source (and the number should be easily recognizable)