[BrachProbablityInfo] Proportional distribution of reachable probabilities

When fixing probability of unreachable edges in
BranchProbabilityInfo::calcMetadataWeights() proportionally distribute
remainder probability over the reachable edges. The old implementation
distributes the remainder probability evenly.
See examples in the fixed tests.

Reviewers: yamauchi, ebrevnov
Tags: #llvm
Differential Revision: https://reviews.llvm.org/D80611