hpvm-clang defaults to O1 optim level

hpvm/tools/hpvm-clang/main.py.in

@@ -229,8 +229,8 @@ See option -b for that."""
         help="[clang emit-llvm] clang++ flags (such as -ffastmath)"
     )
     parser.add_argument(
-        "-O", type=str, default="0", metavar="level",
-        help="[clang emit-llvm] Optimization level"
+        "-O", type=str, default="1", metavar="level",
+        help="[clang emit-llvm] Optimization level. Note that default is -O1."
     )
     parser.add_argument(
         "--std", type=str,