test: add test for issue 15084

The underlying issues have been fixed.
All the individual fixes have their own tests,
but it's still useful to have a plain source test.

Fixes #15084

Change-Id: I06c485a7d0716201bd57d1f3be53668dddd7ec14
Reviewed-on: https://go-review.googlesource.com/22426
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>

test/fixedbugs/issue15084.go

+// compile
+
+// Copyright 2016 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 x
+
+type T struct {
+	i int
+	e interface{}
+}
+
+func (t *T) F() bool {
+	if t.i != 0 {
+		return false
+	}
+	_, ok := t.e.(string)
+	return ok
+}
+
+var x int
+
+func g(t *T) {
+	if t.F() || true {
+		if t.F() {
+			x = 0
+		}
+	}
+}