Support: Add unit tests for BranchProbability

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207540 91177308-0d34-0410-b5e6-96231b3b80d8
Support: Simplify BranchProbability operators

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207541 91177308-0d34-0410-b5e6-96231b3b80d8