cmd/gc: fix error check for self-referential array type.

LGTM=gri, iant
R=gri, iant
CC=golang-codereviews
https://golang.org/cl/75920044

test/fixedbugs/issue7525.go

+// errorcheck
+
+// 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 7525: self-referential array types.
+
+package main
+
+import "unsafe"
+
+var x struct {
+	a [unsafe.Sizeof(x.a)]int // ERROR "array bound|typechecking loop|invalid expression"
+	b [unsafe.Offsetof(x.b)]int // ERROR "array bound"
+	c [unsafe.Alignof(x.c)]int // ERROR "array bound|invalid expression"
+	d [len(x.d)]int // ERROR "array bound|invalid array"
+	e [cap(x.e)]int // ERROR "array bound|invalid array"
+}