test: add a test for gccgo bug #33062.

Gccgo generates incorrect type equality functions for some types.
CL 185817 fixes it. This CL adds a test.

Updates #33062.

Change-Id: Id445c5d44a437512c65c46a029e49b7fc32e4d89
Reviewed-on: https://go-review.googlesource.com/c/go/+/185818
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue33062.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.
+
+// Issue 33062: gccgo generates incorrect type equality
+// functions.
+
+package main
+
+type simpleStruct struct {
+	int
+	string
+}
+
+type complexStruct struct {
+	int
+	simpleStruct
+}
+
+func main() {
+	x := complexStruct{1, simpleStruct{2, "xxx"}}
+	ix := interface{}(x)
+	y := complexStruct{1, simpleStruct{2, "yyy"}}
+	iy := interface{}(y)
+	if ix != ix {
+		panic("FAIL")
+	}
+	if ix == iy {
+		panic("FAIL")
+	}
+}