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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213572 91177308-0d34-0410-b5e6-96231b3b80d8