cmd/compile: schedule in-register OpArg first

OpArgXXXReg values must be scheduled at the very top, as their
registers need to be live at the beginning before any other use
of the register.

Change-Id: Ic76768bb74da402adbe61db3b2d174ecd3f9fffc
Reviewed-on: https://go-review.googlesource.com/c/go/+/306329


Trust: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>