cmd/compile: simplify range expression

Found by running gofmt -s on the file in question.

Change-Id: I84511bd2bc75dff196930a7a87ecf5a2aca2fbb8
Reviewed-on: https://go-review.googlesource.com/64310Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/gc/dcl.go

@@ -1137,7 +1137,7 @@ func checknowritebarrierrec() {
 		// the recursive case, we have to update this at most
 		// len(list) times and can stop when we an iteration
 		// that doesn't change anything.
-		for _ = range list {
+		for range list {
 			c.stable = false
 			for _, n := range list {
 				if n.Func.Pragma&Yeswritebarrierrec != 0 {