[ConstantRange] Use APInt::isNullValue rather than APInt::isMinValue where it would make more sense to thing of 0 as 0 rather than the minimum unsigned value. NFC

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301696 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed