[CodeGen] update code generation optimization level(nfc) (#168190)

Author: zhangtianhao6

llvm/include/llvm/Support/CodeGen.h

@@ -82,7 +82,7 @@ namespace llvm {
   enum class CodeGenOptLevel {
     None = 0,      ///< -O0
     Less = 1,      ///< -O1
-    Default = 2,   ///< -O2, -Os
+    Default = 2,   ///< -O2, -Os, -Oz
     Aggressive = 3 ///< -O3
   };