cmd/compile: add regress test for #20682

Minimal reconstruction of reported failure case.

Manually verified that test fails with CL 45911 reverted.

Change-Id: Ia5d11500d91b46ba1eb5d841db3987edb9136c39
Reviewed-on: https://go-review.googlesource.com/45970
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>

test/fixedbugs/issue20682.dir/p.go

+// Copyright 2017 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 p
+
+import "strings"
+
+type T struct{}
+
+func (T) M() {
+	strings.HasPrefix("", "")
+}

test/fixedbugs/issue20682.dir/q.go

+// Copyright 2017 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 q
+
+import "./p"
+
+type T struct{}
+
+func (T) M() interface{} {
+	return &p.T{}
+}

test/fixedbugs/issue20682.dir/r.go

+// Copyright 2017 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 r
+
+import "./q"
+
+type T struct {
+	q.T
+}

test/fixedbugs/issue20682.go

+// compiledir
+
+// Copyright 2017 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 ignored