cmd/compile: schedule in-register OpArg first
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 <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Run-TryBot: Cherry Zhang <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Than McIntosh <4b2922593166fc595a9b1a408f34f5d3817fe9d2@google.com>

cloudflare/go,golang/go,nawawi/go,cloudflare/go,golang/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,golang/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,CAFxX/go,golang/go