Add a new isSignWrappedSet() method to ConstantRange.

Fix zeroExtend and signExtend to support empty sets, and to return the smallest
possible result set which contains the extension of each element in their
inputs. For example zext i8 [100, 10) to i16 is now [0, 256), not i16 [100, 10)
which contains 63446 members.

llvm-svn: 113187