No longer use register names in I/O space for ldi operands

When disassembling AVRDUDE uses known labels for ldi operations that
initialise a register pair, eg,

      ldi     r30, lo8(nvm.ctrla)  ; 0xcb = 203
      ldi     r31, hi8(nvm.ctrla)  ; 0x01 = 1
      out     cpu.ccp, r24
      st      Z, r18

This is meant to illustrate a possible intention of the code. It is not
unusual to see a base register loaded, for example the USARTC0 register
base, into the Y or Z register pair and read write with displacement from
specific USARTC0 register bytes using the ldd and std opcodes.

However, sometimes these labels are misleading when, eg, the register pair
is just initialised as a counter. In particular that can easily happen for
I/O register addresses for which there are fewer use cases to put them
into a register pair as the in/out opcodes are short and no savings would
be made using ldd/std. For example,

      ldi     r28, lo8(gpio.gpior9) ; 0x09 = 9
      ldi     r29, hi8(gpio.gpior9) ; 0x00 = 0

is most likely used as a 16-bit counter initialised with 9. This would
better be disassembled as

      ldi     r28, 0x09            ; 9
      ldi     r29, 0x00            ; 0

This commit does so by removing the use of registers in I/O space as
symbolic ldi operands when loading a register pair.

Author: stefanrueger

src/disasm.c

@@ -922,8 +922,11 @@ static const char *get_ldi_name(int op1, int op2, Op_context *oxp) {
       if((s = find_symbol(*c, addr*awidth)))
         break;
     if(s && s->name) {          // Label matches the address loaded into register pair
-      s->used = 1;
-      return str_ccprintf("%s%s(%s)", awidth == 2? "pm_": "", ra & 1? "hi8": "lo8", s->name);
+      // Don't use register names in I/O space as ldi label names
+      if(s->type != 'M' || (uint16_t) addr >= 64 + cx->dis_io_offset) {
+        s->used = 1;
+        return str_ccprintf("%s%s(%s)", awidth == 2? "pm_": "", ra & 1? "hi8": "lo8", s->name);
+      }
     }
     if((ra | 1) == 31 && oxp->is_lpm && addr >= cx->dis_start && addr < cx->dis_end) {
       if(cx->dis_pass == 1)