Fix unsound unary fixnum subtraction optimization.

Closes PR13826.

original commit: 93d4a5d4266511166de9e8549afec89922a4eb84

collects/tests/typed-racket/optimizer/tests/unary-fixnum.rkt

@@ -1,10 +1,15 @@
 #;
 (
-TR opt: unary-fixnum.rkt 10:0 (bitwise-not 4) -- unary fixnum
+TR opt: unary-fixnum.rkt 13:0 (bitwise-not 4) -- unary fixnum
+TR opt: unary-fixnum.rkt 14:0 (- (ann 2 Nonnegative-Fixnum)) -- unary fixnum
 -5
+-2
+2
 )
 
 #lang typed/scheme
 #:optimize
 
 (bitwise-not 4)
+(- (ann 2 Nonnegative-Fixnum))
+(- (ann -2 Negative-Fixnum)) ; can't optimize negatives

collects/typed-racket/optimizer/fixnum.rkt

@@ -131,6 +131,8 @@
                   #'(op.unsafe n1.opt n2.opt)))
 
   (pattern (#%plain-app (~and op (~literal -)) f:fixnum-expr)
+           ;; Invalid for `(- <most-negative-fixnum>)'.
+           #:when (subtypeof? #'f -NonNegFixnum)
            #:with opt
            (begin (log-optimization "unary fixnum" fixnum-opt-msg this-syntax)
                   (add-disappeared-use #'op)