test: fix index.go to pass with recent index checks

The compiler now gives an error for out of bounds constant
indexes for arrays, and for negative constant indexes for both
arrays and slices.

With this change the index.go test passes if CLs 6815085,
6815088, and 6812089 are committed.

R=golang-dev, remyoudompheng, rsc
CC=golang-dev
https://golang.org/cl/6810085

test/index.go

@@ -225,7 +225,7 @@ func main() {
 				// the next pass from running.
 				// So run it as a separate check.
 				thisPass = 1
-			} else if i == "i64big" || i == "i64bigger" && runtime.GOARCH == "amd64" {
+			} else if a == "s" && n == "" && (i == "i64big" || i == "i64bigger") && runtime.GOARCH == "amd64" {
 				// On amd64, these huge numbers do fit in an int, so they are not
 				// rejected at compile time.
 				thisPass = 0