Fix a Clang warning in the new NVPTX backend:

In file included from ../lib/Target/NVPTX/VectorElementize.cpp:53:
../lib/Target/NVPTX/NVPTX.h:44:3: warning: default label in switch which covers all enumeration values [-Wcovered-switch-default]
  default: assert(0 && "Unknown condition code");
  ^
1 warning generated.

The prevailing pattern in LLVM is to not use a default label, and instead to
use llvm_unreachable to denote that the switch in fact covers all return paths
from the function.

llvm-svn: 156209