[AArch64] Dump a little more info about unimplemented reg-to-reg copies. NFC

llvm/lib/Target/AArch64/AArch64InstrInfo.cpp

@@ -3621,6 +3621,11 @@ void AArch64InstrInfo::copyPhysReg(MachineBasicBlock &MBB,
     return;
   }
 
+#ifndef NDEBUG
+  const TargetRegisterInfo &TRI = getRegisterInfo();
+  errs() << TRI.getRegAsmName(DestReg) << " = COPY "
+         << TRI.getRegAsmName(SrcReg) << "\n";
+#endif
   llvm_unreachable("unimplemented reg-to-reg copy");
 }