Introduce a new cl::opt to hide "cold" blocks from CFG DOT graphs. Use BFI to get block relative frequency. Hide the block if the frequency is below the threshold set by the command line option value.
This is a generalization of the existing cfg-hide-unreachable-paths and cfg-hide-deoptimize-paths options. Just like with the existing options the main use case is performance analysis. Hiding cold blocks reduces the size of the graph by highlighting the common path.