Back in the day, allp was just a pointer to an array. As a result, the
runtime has a few loops of the form:

    for i := 0; ; i++ {
        p := allp[i]
	if p == nil {
	    break
	}
	...
    }

This is silly now because it requires that allp be one longer than the
maximum possible number of Ps, but now that allp is in Go it has a
length.

Replace these with range loops.

Change-Id: I91ef4bc7bd3c9d4fda2264f4aa1b1d0271d7f578
Reviewed-on: https://go-review.googlesource.com/45571
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>