bytecode compiler: fix coordinate problem in letrec check

Example provided by Jay

pkgs/racket-test-core/tests/racket/optimize.rktl

@@ -6430,4 +6430,31 @@
 
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+(module regression-for-letrec-check-non-app-tracking racket/base
+  (require racket/match)
+
+  (define (j-emit j)
+    (struct :ec (x defs) #:transparent)
+    (struct :def (x e) #:transparent)
+    (define (e->c^ j) '(1 2 3))
+    (define (es->c^ js)
+      (define n (gensym))
+      (match js
+        ['() (e->c '(:con 'void))]
+        [(cons a d)
+         (match-define (:ec ax adefs) (e->c a))
+         (match-define (:ec dx ddefs) (es->c d))
+         (:ec n (list* (:def n 9)
+                       (append ddefs adefs)))]))
+
+    (define ((make-e->c e->c^) j)
+      (match-define (and r (:ec n defs)) (e->c^ j))
+      r)
+    (define e->c (make-e->c e->c^))
+    (define es->c (make-e->c es->c^))
+
+    6))
+
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
 (report-errs)

racket/src/racket/src/letrec_check.c

@@ -312,15 +312,15 @@ static void letrec_check_lets_resume(Letrec_Check_Frame *frame, Scheme_IR_Let_He
        that we have invalidated; i.e., adding check-undefineds around
        references means there is one (more) instance where the LHS
        variable is not used in application position */
-    k = head->count;
+    k = 0;
     for (i = head->num_clauses; i--;) {
       irlv = (Scheme_IR_Let_Value *) body;
-      k -= irlv->count;
       for (j = 0; j < irlv->count; j++) {
         was_checked = (frame->ref[k + j] & LET_CHECKED);
         if (was_checked)
           irlv->vars[j]->non_app_count = irlv->vars[j]->use_count;
       }
+      k += irlv->count;
       body = irlv->body;
     }
   }