Make llvm-flo print dynamic coverage of rewritten functions

Summary:
This is an attempt at determining the hotness of functions we are
rewriting and help detect if we are discarding hot functions. This patch
introduces logic to estimate the number of instructions executed in each
function by using the profile data for branches. It sums the products of
BB frequency and size. Since we can only do this for functions we have
successfully disassembled, created the CFG and annotated with profiling
data, all complex functions that were not disassembled are left out from
this analysis.

(cherry picked from FBD2654985)