In order to be compatible with the form of the atomic instruction in
GAS that accepts the fourth operand as 0 (i.e. am* $rd, $rk, $rj, 0),
we need to treat $rj, 0 as one operand, but only print $rj.
For this, the number of result operands of inline assembly memory
operand ZB constraint is modified to 2 (reg + 0).
Restrictions on register usage in am* instructions have also been
adjusted. When $rd is equal to $r0, the instruction must be
considered legal, because of some special usage like PseudoUNIMP.