[nullsafe][easy] fix the wrong comment about order of arguments in `instanceof`

Summary:
This was introduced in D19770219, and skcho caught it, but for some
reason I seemed to have a blind spot and confused the semantics of `instanceof`.

Reviewed By: skcho

Differential Revision: D19813031

fbshipit-source-id: d939b981b

infer/src/nullsafe/typeCheck.ml

@@ -665,8 +665,8 @@ let rec check_condition_for_sil_prune tenv idenv calls_this find_canonical_dupli
     | _ ->
         None
   in
-  (* check if the expression is coming from (`a` instanceof `b`), and returns `b`, if it is the case *)
+  (* check if the expression is coming from (`a` instanceof `b`), and returns `a`, if it is the case *)
-  let extract_argument_from_instanceof expr =
+  let extract_first_argument_from_instanceof expr =
     match extract_arguments_from_call ComplexExpressions.procname_instanceof expr with
     | Some [argument; _] ->
         Some argument
@@ -805,9 +805,9 @@ let rec check_condition_for_sil_prune tenv idenv calls_this find_canonical_dupli
               accumulated_typestate )
           arguments
     | None -> (
-      match extract_argument_from_instanceof expr with
+      match extract_first_argument_from_instanceof expr with
       | Some argument ->
-          (* ([argument] instanceof [expr] == true) implies (expr != null) *)
+          (* ([argument] instanceof <anything> == true) implies (argument != null) *)
           set_original_pvar_to_nonnull_in_typestate ~with_cond_redundant_check:false argument
             typestate
       | None ->