A better profi rebalancer

This is an extension of **profi** post-processing step that rebalances counts
in CFGs that have basic blocks w/o probes (aka "unknown" blocks). Specifically,
the new version finds many more "unknown" subgraphs and marks more "unknown"
basic blocks as hot (which prevents unwanted optimization passes).

I see up to 0.5% perf on some (large) binaries, e.g., clang-10 and gcc-8.

The algorithm is still linear and yields no build time overhead.