[BranchProbabilityInfo] Handle irreducible loops.

Summary:
Compute the strongly connected components of the CFG and fall back to
use these for blocks that are in loops that are not detected by
LoopInfo when computing loop back-edge and exit branch probabilities.

Reviewers: dexonsmith, davidxl

Subscribers: mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D39385

llvm-svn: 317094