[llvm] [AVR] Remove some custom decoder functions (NFCI) (PR #156852)

[Ben Shi via llvm-commits]

================
@@ -141,18 +106,25 @@ static DecodeStatus decodeCallTarget(MCInst &Inst, unsigned Field,
   return MCDisassembler::Success;
 }
 
-static DecodeStatus decodeFRd(MCInst &Inst, unsigned Insn, uint64_t Address,
-                              const MCDisassembler *Decoder) {
-  unsigned d = fieldFromInstruction(Insn, 4, 5);
-  if (DecodeGPR8RegisterClass(Inst, d, Address, Decoder) ==
-      MCDisassembler::Fail)
-    return MCDisassembler::Fail;
+static DecodeStatus decodeRelCondBrTarget7(MCInst &Inst, unsigned Field,
+                                           uint64_t Address,
+                                           const MCDisassembler *Decoder) {
+  Inst.addOperand(MCOperand::createImm(SignExtend32(Field, 7) << 1));
----------------
benshi001 wrote:

It would be better to add a comment line,
```
The legal range is [-128, 126]
``` 

https://github.com/llvm/llvm-project/pull/156852