Change operands of subtraction expression to have well-defined behaviour.

Change operands of subtraction expression to have well-defined behaviour.

At present, signed arithmetic overflows (i.e. has undefined behaviour) in general, e.g. when computing 0 - INT_MIN or INT_MAX - INT_MIN. The fact that we want the result in the unsigned type does not help us here.

The fix is to convert the operands to the corresponding unsigned type first and then perform the operation in unsigned arithmetic, which is well-defined and has the correct subtraction behaviour.

PiperOrigin-RevId: 193391813