cmd/gc: fix <-<-expr

The temporary-introducing pass was not recursing
into the argumnt of a receive operation.

Fixes #8011.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant, khr
https://golang.org/cl/91540043

src/cmd/gc/order.c

@@ -1053,6 +1053,7 @@ orderexpr(Node **np, Order *order)
 		break;
 
 	case ORECV:
+		orderexpr(&n->left, order);
 		n = ordercopyexpr(n, n->type, order, 1);
 		break;
 	}

test/fixedbugs/issue8011.go

+// run
+
+// 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.
+
+package main
+
+func main() {
+	c := make(chan chan int, 1)
+	c1 := make(chan int, 1)
+	c1 <- 42
+	c <- c1
+	x := <-<-c
+	if x != 42 {
+		println("BUG:", x, "!= 42")
+	}
+}