Add missing identity functions

This change adds four missing identity functions to the identity_funcs list.
These omissions caused certain operands to be incorrectly rendered in the
syntax field of the JSON output.

This update resolves those problems and helps prevent other potential errors
in the JSON generation process.

src/sail_json_backend/json.ml

@@ -483,6 +483,10 @@ let identity_funcs =
     "freg_or_reg_name";
     "vreg_name";
     "maybe_vmask";
+    "sew_flag";
+    "maybe_lmul_flag";
+    "maybe_ta_flag";
+    "maybe_ma_flag";
   ]
 
 let defunction f n =