Fix peval on (call-with-values foo (lambda (x) x))

* module/language/tree-il/peval.scm (peval): Don't
  inline (call-with-values foo (lambda (x) exp)) to (let ((x (foo)))
  exp).  The idea is that call-with-values sets up an explicit context
  in which we are requesting an explicit return arity, and that dropping
  extra values when there's not a rest argument is the wrong thing.
  Fixes #13966.
* test-suite/tests/peval.test ("partial evaluation"): Update test.

module/language/tree-il/peval.scm

@@ -1014,10 +1014,6 @@ top-level bindings from ENV and return the resulting expression."
        ;; reconstruct the let-values, pevaling the consumer.
        (let ((producer (for-values producer)))
          (or (match consumer
-               (($ <lambda-case> src (req-name) #f #f #f () (req-sym) body #f)
-                (for-tail
-                 (make-let src (list req-name) (list req-sym) (list producer)
-                           body)))
                ((and ($ <lambda-case> src () #f rest #f () (rest-sym) body #f)
                      (? (lambda _ (singly-valued-expression? producer))))
                 (let ((tmp (gensym "tmp ")))

test-suite/tests/peval.test

@@ -1354,8 +1354,10 @@
 
   (pass-if-peval
       (call-with-values foo (lambda (x) (bar x)))
-    (let (x) (_) ((call (toplevel foo)))
-         (call (toplevel bar) (lexical x _))))
+    (let-values (call (toplevel foo))
+      (lambda-case
+       (((x) #f #f #f () (_))
+        (call (toplevel bar) (lexical x _))))))
 
   (pass-if-peval
       ((lambda (foo)