cmd/compile: remove unused cases from switch

The first just falls through, and the default case does nothing. They
can be deleted.

Change-Id: I82ab1ce3acde0b8423334cfbf35f9e0c806cd494
Reviewed-on: https://go-review.googlesource.com/65410Reviewed-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/racewalk.go

@@ -553,10 +553,6 @@ func makeaddable(n *Node) {
 		makeaddable(n.Left)
 
 		// nothing to do
-	case ODOTPTR:
-		fallthrough
-	default:
-		break
 	}
 }