cmd/compile: minor cleanup to HashDebugPos

cmd/compile: minor cleanup to HashDebugPos

HashDebugPos function/method included a parameter that was always
the same, and a variable in the same package as the hashdebug code.
So remove it.

(I wrote that code, there was no reason for it to be that way).

Also corrects a stale comment in the loopvar code.

Change-Id: Id3da69cfe6dadeb31d5de62fb76d15103a5d6152
Reviewed-on: https://go-review.googlesource.com/c/go/+/482816
Run-TryBot: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>