repair JIT bug

The bug incorrectly tracks the value that is available in virtual
register R0 --- only in the case that some value was known to be ready
in R0, and some other value was known to be ready in R1, and a value
is moved from R1 to R0.

Closes PR 14523

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

@@ -3436,6 +3436,59 @@
                x)
              exn:fail:contract:variable?)
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Try to check that JIT's register shortcuts
+;; (tracking when a value is ready in a register)
+;; works ok. The example here failed once upon
+;; a time, at least.
+
+;; Example by Dale Vaillancourt
+
+(module check-jit-registers-shortcut racket/base
+  (require racket/fixnum racket/match) 
+
+  (struct bral-empty () #:transparent)
+  (struct bral-node (weight tree rest) #:transparent)
+
+  (struct node (val even odd) #:transparent)
+
+  (struct var (name idx) #:transparent)
+
+
+  (define (half n) (fxrshift n 1))
+
+  (define (lookup-tree w i t)
+    (if (node? t)
+        (if (zero? i) 
+            (node-val t)
+            (let [(w/2 (half w))]
+              (if (<= i w/2)
+                  (lookup-tree w/2 (fx- i 1) (node-even t))
+                  (lookup-tree w/2 (- (- i 1) w/2) (node-odd t)))))
+        (if (zero? i) t #f)))
+
+  (define (lookup i ls)
+    (match ls 
+      [(bral-empty) #f]
+      [(bral-node weight tree ls*)
+       (if (< i weight)
+           (lookup-tree weight i tree)
+           (lookup (- i weight) ls*))]))
+
+  (define a-node
+    (bral-node
+     15
+     (node
+      'a
+      (node 'b (node 'c 'd 'e) (node 'f 'g 'h))
+      (node 'i (node 'j 'k 'l) (node 'm 'n 'o)))
+     (bral-empty)))
+
+  (define result (lookup 2 a-node))
+  (provide result))
+
+(test 'c dynamic-require ''check-jit-registers-shortcut 'result)
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/jit.c

@@ -2035,7 +2035,7 @@ int scheme_generate(Scheme_Object *obj, mz_jit_state *jitter, int is_tail, int w
           if (target != JIT_R1) {
             jit_movr_p(target, JIT_R1);
             if (target == JIT_R0)
-              jitter->r1_status = pos;
+              jitter->r0_status = pos;
             mz_SET_REG_STATUS_VALID(1);
           }
         } else {