cmd/gc: fix typo in ordermapassign

Fixes #7742

LGTM=dave, rsc
R=rsc, bradfitz, dave
CC=golang-codereviews
https://golang.org/cl/85580047

src/cmd/gc/order.c

@@ -472,7 +472,7 @@ ordermapassign(Node *n, Order *order)
 				if(!istemp(m->left))
 					m->left = ordercopyexpr(m->left, m->left->type, order, 0);
 				if(!istemp(m->right))
-					m->right = ordercopyexpr(m->left, m->left->type, order, 0);
+					m->right = ordercopyexpr(m->right, m->right->type, order, 0);
 				l->n = ordertemp(m->type, order, 0);
 				a = nod(OAS, m, l->n);
 				typecheck(&a, Etop);

test/fixedbugs/issue7742.go

+// compile
+
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Issue 7742: cannot use &autotmp_0001 (type *map[string]string) as type *string in function argument
+
+package main
+
+var (
+	m map[string]string
+	v string
+)
+
+func main() {
+	m[v], _ = v, v
+}