test: add test case for issue 8074.

Fixes #8074.

The issue was not reproduceable by revision

go version devel +e0ad7e329637 Thu Jun 19 22:19:56 2014 -0700 linux/arm

But include the original test case in case the issue reopens itself.

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://golang.org/cl/107290043

test/fixedbugs/issue8074.go

+// compile
+
+// Copyright 2014 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 8073.
+// was "cannot take the address of 1"
+
+package main
+
+func main() {
+	a := make([]byte, 10)
+	m := make(map[float64][]byte)
+	go copy(a, m[1.0])
+}