test: add extra test case for issue 7590

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/78040043

test/fixedbugs/issue7590.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 7590: gccgo incorrectly traverses nested composite literals.
+
+package p
+
+type S struct {
+	F int
+}
+
+var M = map[string]S{
+	"a": { F: 1 },
+}
+
+var P = M["a"]
+
+var F = P.F