[X86][NFC] Refine code in X86InstrArithmetic.td

1. Simplify the variable name
2. Change HasOddOpcode to HasEvenOpcode b/c
  a. opcode of any 8-bit arithmetic instruction is even
  b. opcode of a 16/32/64-bit arithmetic instruction is usually
     odd, but it can be even sometimes, e.g. INC/DEC, ADCX/ADOX
  c. so that we can remove `let Opcode = o` for the mentioned corner
     cases.