BPI may trigger signed overflow UB while computing branch probabilities
for cold calls or to unreachables. For example, with our current choice
of weights, we'll crash if there are >= 2^12 branches to an unreachable.
Use an alternate BranchProbability constructor which handles large fractions.
BranchProbablity::getBranchProbablity(..) method takes uint64 as the argument. You can probably use that