[AMDGPU] Fix isSGPRReg for special registers

[critson]

Summary

Special registers, e.g. MODE, do not have register classes so
will cause null pointer exception if passed to isSGPRReg.

[arsenm]

What context is passing this through?

[critson]

In D134025#3795210, @arsenm wrote:

What context is passing this through?

I discovered this while writing a new hazard recognizer which need to check SGPR usage on all VALU instructions.

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

 
 bool SIRegisterInfo::isSGPRReg(const MachineRegisterInfo &MRI,
                                Register Reg) const {
   const TargetRegisterClass *RC;
   if (Reg.isVirtual())
     RC = MRI.getRegClass(Reg);
   else
     RC = getPhysRegClass(Reg);
-  return isSGPRClass(RC);
+  return RC ? isSGPRClass(RC) : false;
 }
 
 const TargetRegisterClass *
 SIRegisterInfo::getEquivalentVGPRClass(const TargetRegisterClass *SRC) const {
   unsigned Size = getRegSizeInBits(*SRC);
   const TargetRegisterClass *VRC = getVGPRClassForBitWidth(Size);
   assert(VRC && "Invalid register class size");
   return VRC;