Add explicit abort so optimized build knows that the function does not return

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2605 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/InstVisitor.h

@@ -101,6 +101,7 @@ struct InstVisitor {
 #include "llvm/Instruction.def"
 
     default: assert(0 && "Unknown instruction type encountered!");
+             abort();
     }
   }