Fix some leftover control reaches end of non-void function warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147874 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/PTX/PTXSubtarget.cpp b/lib/Target/PTX/PTXSubtarget.cpp
index b3b6ffb..aa8f052 100644
--- a/lib/Target/PTX/PTXSubtarget.cpp
+++ b/lib/Target/PTX/PTXSubtarget.cpp
@@ -64,4 +64,5 @@
     case PTX_VERSION_2_2: return "2.2";
     case PTX_VERSION_2_3: return "2.3";
   }
+  llvm_unreachable("Invalid PTX version");
 }