[AMDGPU] use llvm_unreachable instead of default for rp set

[rampitec]

Summary

GCC 9.2 seems to incorrectly issue warning about out of bounds
access. This situation should not happen in any way.

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

if (Idx == AMDGPU::RegisterPressureSets::VGPR_32 ||
       Idx == AMDGPU::RegisterPressureSets::AGPR_32)
     return getRegPressureLimit(&AMDGPU::VGPR_32RegClass,
                                const_cast<MachineFunction &>(MF));
 
   if (Idx == AMDGPU::RegisterPressureSets::SReg_32)
     return getRegPressureLimit(&AMDGPU::SGPR_32RegClass,
                                const_cast<MachineFunction &>(MF));
 
-  return AMDGPUGenRegisterInfo::getRegPressureSetLimit(MF, Idx);
+  llvm_unreachable("Unexpected register pressure set!");
 }
 
 const int *SIRegisterInfo::getRegUnitPressureSets(unsigned RegUnit) const {
   static const int Empty[] = { -1 };
 
   if (hasRegUnit(AMDGPU::M0, RegUnit))
     return Empty;
   return AMDGPUGenRegisterInfo::getRegUnitPressureSets(RegUnit);