bc: fix optimizer bug

Fix inlining problem when inlining has been explicitly disabled.

Closes #3309

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

@@ -6619,6 +6619,28 @@
                   n)])
           (lambda (x) (h x)))))
 
+;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Regression test related to single-use variables
+;; and disabled inlining
+
+(parameterize ([compile-context-preservation-enabled #t]
+               [current-namespace (make-base-namespace)])
+  (void
+   (compile
+    '(module m racket/base
+       (letrec ([f (lambda () 0)]
+                [g (let ([g2
+                          (lambda ()
+                            (let ([x (f)])
+                              (list (lambda () x) h)))])
+                     g2)]
+                [h (letrec ([loop
+                             (lambda ()
+                               (let ([y (g)])
+                                 (list y loop)))])
+                     loop)])
+         h)))))
+
 ;; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (report-errs)

racket/src/racket/src/optimize.c

@@ -9622,7 +9622,7 @@ Scheme_Object *optimize_expr(Scheme_Object *expr, Optimize_Info *info, int conte
             int save_fuel = info->inline_fuel, save_no_types = info->no_types;
             int save_vclock, save_aclock, save_kclock, save_sclock;
             info->size -= 1;
-            info->inline_fuel = 0; /* no more inlining; o->expr was already optimized */
+            info->inline_fuel = -1; /* no more inlining; o->expr was already optimized */
             info->no_types = 1; /* cannot used inferred types, in case `val' inferred them */
             save_vclock = info->vclock; /* allowed to move => no change to clocks */
             save_aclock = info->aclock;