[mips] Eliminate unused code for addrRegReg complex pattern. NFC.

[sdardis]

LGTM, they appear to have been added as part of the initial MSA implementation but were not removed when rL192888 & rL192889 were committed.

lib/Target/Mips/MipsISelDAGToDAG.h

private:
   // Include the pieces autogenerated from the target description.
   #include "MipsGenDAGISel.inc"
 
   // Complex Pattern.
   /// (reg + imm).
   virtual bool selectAddrRegImm(SDValue Addr, SDValue &Base,
                                 SDValue &Offset) const;
 
-  // Complex Pattern.
-  /// (reg + reg).
-  virtual bool selectAddrRegReg(SDValue Addr, SDValue &Base,
-                                SDValue &Offset) const;
-
   /// Fall back on this function if all else fails.
   virtual bool selectAddrDefault(SDValue Addr, SDValue &Base,
                                  SDValue &Offset) const;
 
   /// Match integer address pattern.
   virtual bool selectIntAddr(SDValue Addr, SDValue &Base,
                              SDValue &Offset) const;
 

lib/Target/Mips/MipsISelDAGToDAG.cpp

 /// ComplexPattern used on MipsInstrInfo
 /// Used on Mips Load/Store instructions
 bool MipsDAGToDAGISel::selectAddrRegImm(SDValue Addr, SDValue &Base,
                                         SDValue &Offset) const {
   llvm_unreachable("Unimplemented function.");
   return false;
 }
 
-bool MipsDAGToDAGISel::selectAddrRegReg(SDValue Addr, SDValue &Base,
-                                        SDValue &Offset) const {
-  llvm_unreachable("Unimplemented function.");
-  return false;
-}
-
 bool MipsDAGToDAGISel::selectAddrDefault(SDValue Addr, SDValue &Base,
                                          SDValue &Offset) const {
   llvm_unreachable("Unimplemented function.");
   return false;
 }
 
 bool MipsDAGToDAGISel::selectIntAddr(SDValue Addr, SDValue &Base,
                                      SDValue &Offset) const {

lib/Target/Mips/MipsInstrInfo.td

 // Mips Address Mode! SDNode frameindex could possibily be a match
 // since load and store instructions from stack used it.
 def addr :
   ComplexPattern<iPTR, 2, "selectIntAddr", [frameindex]>;
 
 def addrRegImm :
   ComplexPattern<iPTR, 2, "selectAddrRegImm", [frameindex]>;
 
-def addrRegReg :
-  ComplexPattern<iPTR, 2, "selectAddrRegReg", [frameindex]>;
-
 def addrDefault :
   ComplexPattern<iPTR, 2, "selectAddrDefault", [frameindex]>;
 
 def addrimm10 : ComplexPattern<iPTR, 2, "selectIntAddrMSA", [frameindex]>;
 
 //===----------------------------------------------------------------------===//
 // Instructions specific format
 //===----------------------------------------------------------------------===//

lib/Target/Mips/MipsSEISelDAGToDAG.h

private:
 
   bool selectAddrFrameIndex(SDValue Addr, SDValue &Base, SDValue &Offset) const;
   bool selectAddrFrameIndexOffset(SDValue Addr, SDValue &Base, SDValue &Offset,
                                   unsigned OffsetBits) const;
 
   bool selectAddrRegImm(SDValue Addr, SDValue &Base,
                         SDValue &Offset) const override;
 
-  bool selectAddrRegReg(SDValue Addr, SDValue &Base,
-                        SDValue &Offset) const override;
-
   bool selectAddrDefault(SDValue Addr, SDValue &Base,
                          SDValue &Offset) const override;
 
   bool selectIntAddr(SDValue Addr, SDValue &Base,
                      SDValue &Offset) const override;
 
   bool selectAddrRegImm9(SDValue Addr, SDValue &Base,
                          SDValue &Offset) const;

lib/Target/Mips/MipsSEISelDAGToDAG.cpp

if (Addr.getOpcode() == ISD::ADD) {
     }
   }
 
   return false;
 }
 
 /// ComplexPattern used on MipsInstrInfo
 /// Used on Mips Load/Store instructions
-bool MipsSEDAGToDAGISel::selectAddrRegReg(SDValue Addr, SDValue &Base,
-                                          SDValue &Offset) const {
-  // Operand is a result from an ADD.
-  if (Addr.getOpcode() == ISD::ADD) {
-    Base = Addr.getOperand(0);
-    Offset = Addr.getOperand(1);
-    return true;
-  }
-
-  return false;
-}
-
 bool MipsSEDAGToDAGISel::selectAddrDefault(SDValue Addr, SDValue &Base,
                                            SDValue &Offset) const {
   Base = Addr;
   Offset = CurDAG->getTargetConstant(0, SDLoc(Addr), Addr.getValueType());
   return true;
 }
 
 bool MipsSEDAGToDAGISel::selectIntAddr(SDValue Addr, SDValue &Base,