[BOLT][NFC] Return MCRegister::NoRegister from MCPlusBuilder::getNoRegister()

[yota9]

LGTM, but the clang-format small issue

bolt/include/bolt/Core/MCPlusBuilder.h

public:
   virtual void createInstrIncMemory(InstructionListType &Instrs,
                                     const MCSymbol *Target, MCContext *Ctx,
                                     bool IsLeaf) const {
     llvm_unreachable("not implemented");
   }
 
   /// Return a register number that is guaranteed to not match with
   /// any real register on the underlying architecture.
-  virtual MCPhysReg getNoRegister() const {
-    llvm_unreachable("not implemented");
-  }
+  MCPhysReg getNoRegister() const { return MCRegister::NoRegister; }
 
   /// Return a register corresponding to a function integer argument \p ArgNo
   /// if the argument is passed in a register. Or return the result of
   /// getNoRegister() otherwise. The enumeration starts at 0.
   ///
   /// Note: this should depend on a used calling convention.
   virtual MCPhysReg getIntArgRegister(unsigned ArgNo) const {
     llvm_unreachable("not implemented");

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

bool isRegToRegMove(const MCInst &Inst, MCPhysReg &From,
     To = Inst.getOperand(0).getReg();
     return true;
   }
 
   bool isIndirectCall(const MCInst &Inst) const override {
     return Inst.getOpcode() == AArch64::BLR;
   }
 
-  MCPhysReg getNoRegister() const override { return AArch64::NoRegister; }
-
   bool hasPCRelOperand(const MCInst &Inst) const override {
     // ADRP is blacklisted and is an exception. Even though it has a
     // PC-relative operand, this operand is not a complete symbol reference
     // and BOLT shouldn't try to process it in isolation.
     if (isADRP(Inst))
       return false;
 
     if (isADR(Inst))

bolt/lib/Target/X86/X86MCPlusBuilder.cpp

assert((isCall(Inst) || isBranch(Inst)) && !isIndirectBranch(Inst) &&
            "Invalid instruction");
     Inst.getOperand(0) = MCOperand::createExpr(
         MCSymbolRefExpr::create(TBB, MCSymbolRefExpr::VK_None, *Ctx));
     return true;
   }
 
   MCPhysReg getX86R11() const override { return X86::R11; }
 
-  MCPhysReg getNoRegister() const override { return X86::NoRegister; }
-
   MCPhysReg getIntArgRegister(unsigned ArgNo) const override {
     // FIXME: this should depend on the calling convention.
     switch (ArgNo) {
     case 0:   return X86::RDI;
     case 1:   return X86::RSI;
     case 2:   return X86::RDX;
     case 3:   return X86::RCX;
     case 4:   return X86::R8;