Save ACC low nibble before altered, fix XCHD behaviour.

[cjacker]

Take below codes as example:

  mov R0, #0x20h
  mov A, #0xffh
  mov @R0, #0x11h
  xchd A, @R0

After xchd, Accumlator should be 0xf1 and RAM 0x20h should be 0x1f.

but RAM 0x20h now is '0x11', since low 4 bit of ACC was altered by 'value' before use it to alter the 'value'.