Fix comparison operators on unsigned operands.

8cc miscompiled comparison operators (such as < or >=) that they always
compare operands as signed values. That resulted in getting wrong results
on expressions.

For example, previously, "1U > -1" was evaluated to 1. "1U > -1" is
equivalent to "1U > (unsigned)-1" which in turn equivalent to
"1U > 0xFFFFFFFU", whose result should have been 0.