6g bug: no need for parens around array index expression

where index is a composite literal

R=rsc
CC=golang-dev
https://golang.org/cl/961044

test/bugs/bug267.go

+// $G $D/$F.go || echo BUG
+
+// Copyright 2010 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
+
+type T []int
+
+var a []bool
+
+func _() {
+	if a[T{42}[0]] {
+	}
+	// if (a[T{42}[0]]) {}  // this compiles
+}
+
+/*
+6g bugs/bug267.go
+bugs/bug267.go:14: syntax error: unexpected {, expecting :
+*/

test/golden.out

@@ -185,3 +185,7 @@ panic: barCount != 1
 
 panic PC=xxx
 BUG
+
+=========== bugs/bug267.go
+bugs/bug267.go:14: syntax error: unexpected {, expecting :
+BUG