cmd/compile: fold IsInBounds with small index

cmd/compile: fold IsInBounds with small index

For the following example, but there are a few more in the stdlib:
func histogram(b []byte, h *[256]int32) {
        for _, t := range b {
                h[t]++
        }
}

Change-Id: I56615f341ae52e02ef34025588dc6d1c52122295
Reviewed-on: https://go-review.googlesource.com/20924
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Alexandru Moșoi <alexandru@mosoi.ro>
TryBot-Result: Gobot Gobot <gobot@golang.org>