Fix the condition in this assertion, and also make it into an unreachable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@226843 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DebugInfo.h

@@ -904,7 +904,7 @@ public:
       case dwarf::DW_OP_plus:  std::advance(I, 2); break;
       case dwarf::DW_OP_deref: std::advance(I, 1); break;
       default:
-        assert("unsupported operand");
+        llvm_unreachable("unsupported operand");
       }
     }
   };