Constant-folding eq? and eqv? uses deduplication

* test-suite/tests/peval.test ("partial evaluation"): Add tests.
* module/language/tree-il/peval.scm (peval): Constant-fold eq? and eqv?
  using equal?, anticipating deduplication.

module/language/tree-il/peval.scm

@@ -479,7 +479,15 @@ top-level bindings from ENV and return the resulting expression."
         (lambda ()
           (call-with-values
               (lambda ()
-                (apply (module-ref the-scm-module name) args))
+                (case name
+                  ((eq? eqv?)
+                   ;; Constants will be deduplicated later, but eq?
+                   ;; folding can happen now.  Anticipate the
+                   ;; deduplication by using equal? instead of eq?.
+                   ;; Same for eqv?.
+                   (apply equal? args))
+                  (else
+                   (apply (module-ref the-scm-module name) args))))
             (lambda results
               (values #t results))))
         (lambda _

test-suite/tests/peval.test

@@ -1321,4 +1321,12 @@
           (if (apply (primitive pair?) (toplevel arg))
               (set! (lexical l _) (toplevel arg))
               (void))
-          (apply (primitive @apply) (toplevel f) (lexical l _))))))
+          (apply (primitive @apply) (toplevel f) (lexical l _)))))
+
+  (pass-if-peval
+      (eq? '(a b) '(a b))
+    (const #t))
+
+  (pass-if-peval
+      (eqv? '(a b) '(a b))
+    (const #t)))