test: add some tests that gccgo failed to handle correctly

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5671090

test/fixedbugs/bug419.go

+// compile
+
+// 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 1811.
+// gccgo failed to compile this.
+
+package p
+
+type E interface{}
+
+type I interface {
+	E
+	E
+}

test/fixedbugs/bug420.go

+// compile
+
+// 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 1757.
+// gccgo failed to compile this.
+
+package main
+
+func main() {
+     (_) = 0
+}

test/fixedbugs/bug421.go

+// errorcheck
+
+// 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 1927.
+// gccgo failed to issue the first error below.
+
+package main
+
+func main() {
+	println(int(1) == uint(1))	// ERROR "types"
+	var x int = 1
+	var y uint = 1
+	println(x == y)			// ERROR "types"
+}

test/fixedbugs/bug422.go

+// compile
+
+// 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.
+
+// gccgo crashed compiling this file.
+
+package p
+
+var V = "a" > "b"