[BOLT] Explicitly check for unreachable basic blocks in gadget scanner

In an optimized code, unreachable basic block is likely to indicate an error in CFG reconstruction performed by BOLT's core. As this can degrade analysis quality, PtrAuth analyses implemented in `llvm-bolt-binary-analysis` warn the user about such conditions in `FunctionAnalysisContext::findUnsafeUses()` along other preparations to running gadget detectors.

As soon as #170973 is resolved (either by #176135 or by any other patch), it seems natural to skip `findUnsafeUses` altogether if only authentication oracles are searched for. In that case it would be reasonable to implement this check for unreachable basic blocks as a yet another option for `--scanners=...` argument - instead of a full-blown dataflow analysis, a simple BFS should be enough to identify exactly the same set of basic blocks (those having no predecessors at all, as well as those belonging to an unreachable loop).