test: add bug494: test case miscompiled by gccgo

Gccgo incorrectly executed functions multiple times when they appeared
in a composite literal that required a conversion between different
interface types.

Change-Id: I7b40e76ed23fa8440ffa03b262041265c109adf7
Reviewed-on: https://go-review.googlesource.com/1710Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

test/fixedbugs/bug494.go

+// run
+
+// Copyright 2014 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 incorrectly executed functions multiple times when they
+// appeared in a composite literal that required a conversion between
+// different interface types.
+
+package main
+
+type MyInt int
+
+var c MyInt
+
+func (c *MyInt) S(i int) {
+	*c = MyInt(i)
+}
+
+func (c *MyInt) V() int {
+	return int(*c)
+}
+
+type i1 interface {
+	S(int)
+	V() int
+}
+
+type i2 interface {
+	V() int
+}
+
+type s struct {
+	i i2
+}
+
+func f() i1 {
+	c++
+	return &c
+}
+
+func main() {
+	p := &s{f()}
+	if v := p.i.V(); v != 1 {
+		panic(v)
+	}
+	if c != 1 {
+		panic(c)
+	}
+}