what ConstantRange does for unsigned integers. Factor out a
common base class for common functionality.
Add some new functions for performing arithmetic on constant
ranges. Some of these are currently just stubbed out with
conservative implementations.
Add unittests for ConstantRange and ConstantSignedRange.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@75177 91177308-0d34-0410-b5e6-96231b3b80d8
