Flags for displaying only hot nodes in CFGPrinter graph

Added two flags to omit uncommon or dead paths in the CFG graphs:
  -cfg-hide-unreachable-paths
  -cfg-hide-deoptimize-paths

The main purpose is performance analysis when such block are not
"interesting" from perspective of common path performance.

Reviewed By: apilipenko, davidxl

Differential Revision: https://reviews.llvm.org/D74346