cmd/compile: lock-in test for repeated variables in range declaration

Fixes #6772.

Lock-in test for invalid range loop: repeated variables in range declaration.

Change-Id: I37dd8b1cd7279abe7810deaf8a5d485c5c3b73ca
Reviewed-on: https://go-review.googlesource.com/34714Reviewed-by: Keith Randall <khr@golang.org>

test/fixedbugs/issue6772.go

+// errorcheck
+
+// 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 p
+
+func f1() {
+	for a, a := range []int{1, 2, 3} { // ERROR "a repeated on left side of :="
+		println(a)
+	}
+}
+
+func f2() {
+	var a int
+	for a, a := range []int{1, 2, 3} { // ERROR "a repeated on left side of :="
+		println(a)
+	}
+	println(a)
+}