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://codereview.appspot.com/12311046