Now the registration phase looks like:

    var cases [4]runtime.scases
    var order [8]uint16
    selectsend(&cases[0], c1, &v1)
    selectrecv(&cases[1], c2, &v2, nil)
    selectrecv(&cases[2], c3, &v3, &ok)
    selectdefault(&cases[3])
    chosen := selectgo(&cases[0], &order[0], 4)

Primarily, this is just preparation for having the compiler open-code
selectsend, selectrecv, and selectdefault.

As a minor benefit, order can now be layed out separately on the stack
in the pointer-free segment, so it won't take up space in the
function's stack pointer maps.

Change-Id: I5552ba594201efd31fcb40084da20b42ea569a45
Reviewed-on: https://go-review.googlesource.com/37933
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>