[dev.inline] cmd/compile/internal/syntax: simplified position code

Reviewed in and cherry-picked from https://go-review.googlesource.com/#/c/33805/.

Change-Id: I859d9bd5f2256ca78f7b24b330290f7ae600854d
Reviewed-on: https://go-review.googlesource.com/34234


Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>