cmd/compile: open code select{send,recv,default}

cmd/compile: open code select{send,recv,default}

Registration now looks like:

        var cases [4]runtime.scases
        var order [8]uint16
	cases[0].kind = caseSend
	cases[0].c = c1
	cases[0].elem = &v1
	if raceenabled || msanenabled {
		selectsetpc(&cases[0])
	}
	cases[1].kind = caseRecv
	cases[1].c = c2
	cases[1].elem = &v2
	if raceenabled || msanenabled {
		selectsetpc(&cases[1])
	}
	...

Change-Id: Ib9bcf426a4797fe4bfd8152ca9e6e08e39a70b48
Reviewed-on: https://go-review.googlesource.com/37934
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>