[sledge] Optimize Trace enabled checking

Summary:
If either all or no tracing is enabled, it is not necessary to compute
the module and function names for each Trace operation to determine
whether or not it is enabled. This diff optimizes this case.

Reviewed By: ngorogiannis

Differential Revision: D27564869

fbshipit-source-id: bf766eb2f

sledge/ppx_trace/trace/trace.ml

@@ -169,8 +169,10 @@ let split_mod_fun_name s =
   (mod_name, fun_name)
 
 let enabled mod_fun_name =
-  let mod_name, fun_name = split_mod_fun_name mod_fun_name in
   let {trace_all; trace_mods_funs; _} = !config in
+  if Map.is_empty trace_mods_funs then trace_all
+  else
+    let mod_name, fun_name = split_mod_fun_name mod_fun_name in
     match Map.find mod_name trace_mods_funs with
     | {trace_mod; trace_funs} -> (
       try Map.find fun_name trace_funs