Implement unreachable BB elimination in llvm-flo

Summary:
It is important to remove dead blocks to free up space in functions
and allow us to reorder blocks or align branch targets with more
freedom. This patch implements a simple algorithm to delete all basic
blocks that are not reachable from the entry point. Note that C++
exceptions may create "unreachable" blocks, so this option must be
used with care.

(cherry picked from FBD2562637)