R600: silence GCC warning

GCC believes it may be possible to not return a value from the switch:
  lib/Target/R600/SIRegisterInfo.cpp:187:1: warning: control reaches end of non-void function [-Wreturn-type]

Add an unreachable label to indicate that this is not possible and still permit
switch coverage checking.

llvm-svn: 213572

llvm/lib/Target/R600/SIRegisterInfo.cpp

@@ -184,4 +184,5 @@ unsigned SIRegisterInfo::getPreloadedValue(const MachineFunction &MF,
   case SIRegisterInfo::SCRATCH_PTR:
     return AMDGPU::SGPR2_SGPR3;
   }
+  llvm_unreachable("unexpected preloaded value type");
 }