repair to cp0 changes for single-value checking

original commit: 1d0d02a1fd01629a2b72b82e95b60084f5bff4fb

mats/cp0.ms

@@ -2815,5 +2815,8 @@
                                            (fields x y))
                                          (if (make-posn 0 ,e) ,v 'other)))
                         posn-decl-expanded)
-   
+   (parameterize ([enable-cp0 #t] [#%$suppress-primitive-inlining #f])
+     (equivalent-expansion?
+      (expand/optimize '(let ([g1 (begin (unknown) (void))]) 10))
+      '(begin (unknown) 10)))
 )

s/cp0.ss

@@ -737,7 +737,10 @@
             (cond
              [(and (eq? ctxt 'effect) (simple? e2))
               e1]
-             [(and (eq? ctxt 'ignored) (simple1? e2))
+             [(and (eq? ctxt 'ignored) (simple1? e2)
+                   ;; don't move e1 into a single-value
+                   ;; position unless that's ok
+                   (single-valued? e1))
               e1]
              [else
               (let ([e1 (nanopass-case (Lsrc Expr) e1