cmd/compile: schedule SP earlier

cmd/compile: schedule SP earlier

The actual scheduling of SP early doesn't really matter, but lots of
early spills (of arguments) depend on SP so they can't schedule until
SP does.

Fixes #58482

Change-Id: Ie581fba7cb173d665c11f797f39d824b1c040a2d
Reviewed-on: https://go-review.googlesource.com/c/go/+/468455
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Alessandro Arzilli <alessandro.arzilli@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>