test: add extra test for issue18661

Make sure that the lack of an lvalue doesn't
cause extra side-effects.

Updates #18661
Updates #18739

Change-Id: I52eb4b4a5c6f8ff5cddd2115455f853c18112c19
Reviewed-on: https://go-review.googlesource.com/36126
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

test/fixedbugs/issue18661.go

-// compile
+// run
 
 // 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
+package main
+
+import (
+	"fmt"
+	"os"
+)
 
 var (
 	e interface{}
@@ -16,3 +21,19 @@ func test(obj interface{}) {
 	if obj != struct{ a *string }{} {
 	}
 }
+
+var x int
+
+func f() [2]string {
+	x++
+	return [2]string{"abc", "def"}
+}
+
+func main() {
+	var e interface{} = [2]string{"abc", "def"}
+	_ = e == f()
+	if x != 1 {
+		fmt.Println("x=", x)
+		os.Exit(1)
+	}
+}