test: add test that was miscompiled by gccgo

For #49512

Change-Id: Ic08652a4ec611b27150bf10b1118c1395715e5d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/363156
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

test/fixedbugs/issue49512.go

@@ -0,0 +1,54 @@
+// run
+
+// Copyright 2021 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.
+
+package main
+
+type S struct{
+	m1Called, m2Called bool
+}
+
+func (s *S) M1(int) (int, int) {
+	s.m1Called = true
+	return 0, 0
+}
+
+func (s *S) M2(int) (int, int) {
+	s.m2Called = true
+	return 0, 0
+}
+
+type C struct {
+	calls []func(int) (int, int)
+}
+
+func makeC() Funcs {
+	return &C{}
+}
+
+func (c *C) Add(fn func(int) (int, int)) Funcs {
+	c.calls = append(c.calls, fn)
+	return c
+}
+
+func (c *C) Call() {
+	for _, fn := range c.calls {
+		fn(0)
+	}
+}
+
+type Funcs interface {
+	Add(func(int) (int, int)) Funcs
+	Call()
+}
+
+func main() {
+	s := &S{}
+	c := makeC().Add(s.M1).Add(s.M2)
+	c.Call()
+	if !s.m1Called || !s.m2Called {
+		panic("missed method call")
+	}
+}