Now that ConstantInt::isValueValidForType can handle signed and unsigned

Now that ConstantInt::isValueValidForType can handle signed and unsigned
values regardless of the signedness of the constant's type, it is okay to
always make the AsmWriter.cpp print constant ints as signed values. The
AsmParser will automatically handle things like: uint -1 as a result.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@32686 91177308-0d34-0410-b5e6-96231b3b80d8