[BPI] Use a safer constructor to calculate branch probabilities

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 a safer BranchProbability constructor which is better at handling
fractions with large denominators.

rdar://problem/29368161

Differential Revision: https://reviews.llvm.org/D27862

llvm-svn: 290016