[dev.unified] cmd/compile/internal/noder: prefer *At functions

[dev.unified] cmd/compile/internal/noder: prefer *At functions

Unified IR tries to avoid depending on base.Pos, so we should prefer
explicit position arguments wherever possible.

Change-Id: I7163b1b8c5244fe7c2a7989e6a3f459a21a23e81
Reviewed-on: https://go-review.googlesource.com/c/go/+/418096
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>