Support: Add BranchProbability::scale() and ::scaleByInverse()

Add API to `BranchProbability` for scaling big integers.  Next job is to
rip the logic out of `BlockMass` and `BlockFrequency`.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207544 91177308-0d34-0410-b5e6-96231b3b80d8
Support: remove unnecessary namespace

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207545 91177308-0d34-0410-b5e6-96231b3b80d8