test: add test for conversion of untyped bool to interface

gccgo miscompiled this case.

Updates #40152

Change-Id: I8448c155e802e39d8fc7cda4930ce62cb6363ce5
Reviewed-on: https://go-review.googlesource.com/c/go/+/242000
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

test/fixedbugs/issue40152.go

@@ -0,0 +1,21 @@
+// run
+
+// Copyright 2020 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.
+
+// Gccgo mishandles converting an untyped boolean to an interface type.
+
+package main
+
+func t(args ...interface{}) bool {
+        x := true
+        return x == args[0]
+}
+
+func main() {
+	r := t("x" == "x" && "y" == "y")
+	if !r {
+		panic(r)
+	}
+}