x86: make swap_2_operands() have unsigned parameters

x86: make swap_2_operands() have unsigned parameters

All callers pass unsigned values (in some cases by virtue of passing
non-negative literal numbers).

This in turn requires struct {Mask,RC,Broadcast}_Operation's "operand"
fields to become unsigned, in turn allowing to reduce the amount of
casting needed (the two new casts that are necessary cast _to_
"unsigned" instead of _from_, as that's the form that'll never case
undefined behavior).