cmd/compile/internal: decouple scope tracking from location lists

We're trying to enable location lists by default, and it's easier to do
that if we don't have to worry about scope tracking at the same time.
We can evaluate their performance impact separately.

However, that does mean that "err" is ambiguous in the test case, so
rename it to err2 for now.

Change-Id: I24f119016185c52b7d9affc74207f6a5b450fb6f
Reviewed-on: https://go-review.googlesource.com/89355


Run-TryBot: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>