cmd/compile: add test for Issue 12536

The fixedbugs/issue12536.go file was erroneously deleted just before
committing the patch that fixed the issue (CL 14400).

That's an easy test and there's a small reproducer in the issue, add
it back.

Updates #12536

Change-Id: Ib7b0cd245588299e9a5469e1d75805fd0261ce1a
Reviewed-on: https://go-review.googlesource.com/40712Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

test/fixedbugs/issue12536.go

+// compile
+
+// 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.
+
+// Issue 12536: compiler crashes while checking keys in a map literal for equality
+
+package p
+
+func main() {
+	m1 := map[interface{}]interface{}{
+		nil:  0,
+		true: 1,
+	}
+	m2 := map[interface{}]interface{}{
+		true: 1,
+		nil:  0,
+	}
+	println(len(m1))
+	println(len(m2))
+}