cmd/compile: cleanup uses of ir.Orig and ir.SepCopy

cmd/compile: cleanup uses of ir.Orig and ir.SepCopy

Mostly automated refactoring with gofmt:

gofmt -r 'ir.Orig(n) -> n'
gofmt -r 'ir.SepCopy(n) -> ir.Copy(n)'

Followed by some manual cleanups.

Change-Id: Ib35abeba9e60b70ba463e161fb39358fb058a83e
Reviewed-on: https://go-review.googlesource.com/c/go/+/526398
Reviewed-by: Keith Randall <khr@golang.org>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>