test: add test for gccgo bug #26248

The fix is CL 122756.

Updates #26248.

Change-Id: Ic4250ab5d01da9f65d0bc033e2306343d9c87a99
Reviewed-on: https://go-review.googlesource.com/122757
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue26248.go

+// run
+
+// Copyright 2018 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 26248: gccgo miscompiles interface field expression.
+// In G().M where G returns an interface, G() is evaluated twice.
+
+package main
+
+type I interface {
+	M()
+}
+
+type T struct{}
+
+func (T) M() {}
+
+var g = 0
+
+//go:noinline
+func G() I {
+	g++
+	return T{}
+}
+
+//go:noinline
+func Use(interface{}) {}
+
+func main() {
+	x := G().M
+	Use(x)
+
+	if g != 1 {
+		println("want 1, got", g)
+		panic("FAIL")
+	}
+}