cmd/compile: added test to ensure that accidental fix remains

Bug 15141 was apparently fixed by some other change to the
compiler (this is plausible, it was a weird bug dependent
on a particular way of returning a large named array result),
add the test to ensure that it stays fixed.

Updates #15141.

Change-Id: I3d6937556413fab1af31c5a1940e6931563ce2f3
Reviewed-on: https://go-review.googlesource.com/31972
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

test/fixedbugs/issue15141.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 main
+
+func main() {
+	a := f(1, 99)
+	b := g(0xFFFFFFe, 98)
+	c := h(0xFFFFFFe, 98)
+	println(a[1], b[1], c[1], a[0xFFFFFFe], b[0xFFFFFFe], c[0xFFFFFFe])
+}
+
+//go:noinline
+func f(i, y int) (a [0xFFFFFFF]byte) {
+	a[i] = byte(y)
+	return
+}
+
+//go:noinline
+func g(i, y int) [0xFFFFFFF]byte {
+	var a [0xFFFFFFF]byte
+	a[i] = byte(y)
+	return a
+}
+
+//go:noinline
+func h(i, y int) (a [0xFFFFFFF]byte) {
+	a[i] = byte(y)
+	return a
+}