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

Reviewed By: yota9

Differential Revision: https://reviews.llvm.org/D120863

bolt/include/bolt/Core/MCPlusBuilder.h

@@ -424,9 +424,7 @@ public:
 
   /// 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

bolt/lib/Target/AArch64/AArch64MCPlusBuilder.cpp

@@ -204,8 +204,6 @@ public:
     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

bolt/lib/Target/X86/X86MCPlusBuilder.cpp

@@ -3202,8 +3202,6 @@ public:
 
   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) {