cmd/gc: fix inlining bug with receive operator.

The receive operator was given incorrect precedence
resulting in incorrect deletion of parentheses.

Fixes #3843.

R=rsc
CC=golang-dev, remy
https://golang.org/cl/6442049

src/cmd/gc/fmt.c

@@ -964,7 +964,6 @@ static int opprec[] = {
 	[OPAREN] = 8,
 	[OPRINTN] = 8,
 	[OPRINT] = 8,
-	[ORECV] = 8,
 	[ORUNESTR] = 8,
 	[OSTRARRAYBYTE] = 8,
 	[OSTRARRAYRUNE] = 8,
@@ -996,6 +995,7 @@ static int opprec[] = {
 	[OMINUS] = 7,
 	[OADDR] = 7,
 	[OIND] = 7,
+	[ORECV] = 7,
 
 	[OMUL] = 6,
 	[ODIV] = 6,

test/fixedbugs/bug448.dir/pkg1.go

+// Copyright 2012 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 pkg1
+
+var x = make(chan interface{})
+
+func Do() int {
+	return (<-x).(int)
+}

test/fixedbugs/bug448.dir/pkg2.go

+// Copyright 2012 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 3843: inlining bug due to wrong receive operator precedence.
+
+package pkg2
+
+import "./pkg1"
+
+func F() {
+	pkg1.Do()
+}
+

test/fixedbugs/bug448.go

+// compiledir
+
+// Copyright 2012 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 ignored