merge the new functionality and unittests into ConstantRange. Thanks to
Nick Lewycky for pointing out that it isn't necessary to have two separate
classes here.

llvm-svn: 75191

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.

llvm-svn: 75177