[AVR] Adjust to Register class change

[aykevl]

Summary

A refactor in r364191 changed register types from an unsigned int to the llvm:Register class. Adjust the AVR backend to this change.

This fixes build errors when building with the experimental AVR backend enabled.

llvm/lib/Target/AVR/AVRRegisterInfo.h

public:
   getLargestLegalSuperClass(const TargetRegisterClass *RC,
                             const MachineFunction &MF) const override;
 
   /// Stack Frame Processing Methods
   void eliminateFrameIndex(MachineBasicBlock::iterator MI, int SPAdj,
                            unsigned FIOperandNum,
                            RegScavenger *RS = NULL) const override;
 
-  unsigned getFrameRegister(const MachineFunction &MF) const override;
+  Register getFrameRegister(const MachineFunction &MF) const override;
 
   const TargetRegisterClass *
   getPointerRegClass(const MachineFunction &MF,
                      unsigned Kind = 0) const override;
 
   /// Splits a 16-bit `DREGS` register into the lo/hi register pair.
   /// \param Reg A 16-bit register to split.
   void splitReg(unsigned Reg, unsigned &LoReg, unsigned &HiReg) const;

llvm/lib/Target/AVR/AVRRegisterInfo.cpp

if (Offset > 62) {
     Offset = 62;
   }
 
   MI.getOperand(FIOperandNum).ChangeToRegister(AVR::R29R28, false);
   assert(isUInt<6>(Offset) && "Offset is out of range");
   MI.getOperand(FIOperandNum + 1).ChangeToImmediate(Offset);
 }
 
-unsigned AVRRegisterInfo::getFrameRegister(const MachineFunction &MF) const {
+Register AVRRegisterInfo::getFrameRegister(const MachineFunction &MF) const {
   const TargetFrameLowering *TFI = MF.getSubtarget().getFrameLowering();
   if (TFI->hasFP(MF)) {
     // The Y pointer register
     return AVR::R28;
   }
 
   return AVR::SP;
 }