[AMDGPU] use llvm_unreachable instead of default for rp set

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

Differential Revision: https://reviews.llvm.org/D75071

llvm/lib/Target/AMDGPU/SIRegisterInfo.cpp

@@ -1820,7 +1820,7 @@ unsigned SIRegisterInfo::getRegPressureSetLimit(const MachineFunction &MF,
     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 {