Support: Move class ScaledNumber

ScaledNumber has been cleaned up enough to pull out of BFI now.  Still
work to do there (tests for shifting, bloated printing code, etc.), but
it seems clean enough for its new home.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211562 91177308-0d34-0410-b5e6-96231b3b80d8