test: add a test for gccgo bug in handling break statement in a select

Gccgo CL 184998 added optimizations for one- and two-case select
statements. But it didn't handle break statement in the select
case correctly. The fix is CL 185519. This CL adds a test.

Change-Id: Ide1b199f106172b41dd77c1f6e0d662fccdd8cc5
Reviewed-on: https://go-review.googlesource.com/c/go/+/185520
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

test/chan/select8.go

+// run
+
+// Copyright 2019 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.
+
+// Test break statements in a select.
+// Gccgo had a bug in handling this.
+// Test 1,2,3-case selects, so it covers both the general
+// code path and the specialized optimizations for one-
+// and two-case selects.
+
+package main
+
+var ch = make(chan int)
+
+func main() {
+	go func() {
+		for {
+			ch <- 5
+		}
+	}()
+
+	select {
+	case <-ch:
+		break
+		panic("unreachable")
+	}
+
+	select {
+	default:
+		break
+		panic("unreachable")
+	}
+
+	select {
+	case <-ch:
+		break
+		panic("unreachable")
+	default:
+		break
+		panic("unreachable")
+	}
+
+	select {
+	case <-ch:
+		break
+		panic("unreachable")
+	case ch <- 10:
+		panic("unreachable")
+	default:
+		break
+		panic("unreachable")
+	}
+}