cmd/gc: get rid of redundant slice bound check.

For normal slices a[i:j] we're generating 3 bounds
checks: j<={len(string),cap(slice)}, j<=j (!), and i<=j.
Somehow snuck in as part of the [i:j:k] implementation
where the second check does something.
Remove the second check when we don't need it.

R=rsc, r
CC=golang-dev
https://golang.org/cl/12311046

src/cmd/gc/walk.c

@@ -2623,7 +2623,7 @@ sliceany(Node* n, NodeList **init)
 		cb = n->right->right->right;
 	} else {
 		hb = n->right->right;
-		cb = hb;
+		cb = N;
 	}
 
 	bounded = n->etype;