test: add a test that gccgo miscompiled

Updates #20923

Change-Id: Ia1210ea3dec39e5db2521aeafca24d6e731f0c93
Reviewed-on: https://go-review.googlesource.com/91657
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue20923.go

+// compile
+
+// Copyright 2018 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 20923: gccgo failed to compile parenthesized select case expressions.
+
+package p
+
+func F(c chan bool) {
+	select {
+	case (<-c):
+	case _ = (<-c):
+	case _, _ = (<-c):
+	case (c) <- true:
+	default:
+	}
+}