[PGO] Use the sum of profile counts to fix the function entry count

Raw profile count values for each BB are not kept after profile
annotation. We record function entry count and branch weights
and use them to compute the count when needed.  This mechanism
works well in a perfect world, but often breaks in real programs,
because of number prevision, inconsistent profile, or bugs in
BFI). This patch uses sum of profile count values to fix
function entry count to make the BFI count close to real profile
counts.

Differential Revision: https://reviews.llvm.org/D61540