[AVR] Remove LPM/ELPM custom decoder (NFC) (#156928)

Follow-up to #156361.

Author: s-barannikov

llvm/lib/Target/AVR/AVRInstrFormats.td

@@ -201,20 +201,14 @@ class FSTLD<bit type, bits<2> mode, dag outs, dag ins, string asmstr,
 //===---------------------------------------------------------------------===//
 class FLPMX<bit e, bit p, dag outs, dag ins, string asmstr, list<dag> pattern>
     : AVRInst16<outs, ins, asmstr, pattern> {
+  bits<0> z;
   bits<5> rd;
 
-  let Inst{15 - 12} = 0b1001;
-
-  let Inst{11 - 9} = 0b000;
-  let Inst{8} = rd{4};
-
-  let Inst{7 - 4} = rd{3 - 0};
-
+  let Inst{15 - 9} = 0b1001000;
+  let Inst{8 - 4} = rd;
   let Inst{3 - 2} = 0b01;
   let Inst{1} = e;
   let Inst{0} = p;
-
-  let DecoderMethod = "decodeFLPMX";
 }
 
 //===----------------------------------------------------------------------===//

llvm/lib/Target/AVR/Disassembler/AVRDisassembler.cpp

@@ -150,14 +150,6 @@ static DecodeStatus decodeFRd(MCInst &Inst, unsigned Insn, uint64_t Address,
   return MCDisassembler::Success;
 }
 
-static DecodeStatus decodeFLPMX(MCInst &Inst, unsigned Insn, uint64_t Address,
-                                const MCDisassembler *Decoder) {
-  if (decodeFRd(Inst, Insn, Address, Decoder) == MCDisassembler::Fail)
-    return MCDisassembler::Fail;
-  Inst.addOperand(MCOperand::createReg(AVR::R31R30));
-  return MCDisassembler::Success;
-}
-
 static DecodeStatus decodeFFMULRdRr(MCInst &Inst, unsigned Insn,
                                     uint64_t Address,
                                     const MCDisassembler *Decoder) {