optimizer: reduce constants to #t/#f in boolean contexts

While something like `(if 7 e1 e2)` was already reduced to `e1`,
this improvement makes `(if (let ([x (random)]) 7) e1 e2)`
reduce to `(if (let ([x (random)]) #t) e1 e2)`.

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

@@ -1328,6 +1328,18 @@
                              (a1)
                              (a2))))
 
+(test-comp '(if (let ([z (random)]) null) 1 2)
+           '(if (let ([z (random)]) #t) 1 2))
+
+(test-comp '(if (if (list? (cons 1 null)) null (void)) 1 2)
+           '1)
+
+(test-comp '(if (if (list? (cons 1 null)) 7 8) 1 2)
+           '1)
+
+(test-comp '(if (if (list? (cons 1 null)) #t #t) 1 2)
+           '1)
+
 (test-comp '(lambda (y)
               (let ([f (lambda (x) x)])
                 (if f

racket/src/racket/src/optimize.c

@@ -6409,7 +6409,12 @@ Scheme_Object *scheme_optimize_expr(Scheme_Object *expr, Optimize_Info *info, in
     return module_optimize(expr, info, context);
   default:
     info->size += 1;
-    return expr;
+    if ((context & OPT_CONTEXT_BOOLEAN)
+        && (SCHEME_TYPE(expr) > _scheme_compiled_values_types_)
+        && SCHEME_TRUEP(expr))
+      return scheme_true;
+    else
+      return expr;
   }
 }