Teach llvm-flo how to reorder basic blocks with a heuristic

Summary:
This patch introduces a first approach to reorder basic blocks based on
profiling data that gives us the execution frequency for each edge. Our strategy
is to layout basic blocks in a order that maximizes the weight (hotness) of
branches that will be deleted. We can delete branches when src comes right
before dst in the new layout order. This can be reduced to the TSP problem. This
patch uses a greedy heuristic to solve the problem: we start with a graph with
no edges and progressively add edges by choosing the hottest edges first,
building a layout order that attempts to put BBs with hot edges together.

(cherry picked from FBD2544076)