SBX SBX Accu AND X-Register, subtract operand, result into X-Register SBX

                                                        N V - B D I Z C
  Operation:  X <- (X)/\(A) - M                         / . . . . . / /

  +----------------+-----------------------+---------+---------+----------+
  | Addressing Mode| Assembly Language Form| OP CODE |No. Bytes|No. Cycles|
  +----------------+-----------------------+---------+---------+----------+
  |  Immediate     |   SBX #$FF            |   $CB   |    2    |    2     |
  +----------------+-----------------------+---------+---------+----------+

  Note: Performs CMP and DEX at the same time, so the subtraction
  ignores flags 'C' and 'D' (like CMP, unlike SBC).

  Also known as AXS.