cmd/compile: Avoid and filter out zero-length location-lists.

This change avoids creating zero length location lists by
repairing an overly aggressive change in CL146718
and by explicitly checking for and filtering out any
zero-length lists that are detected (building
compiler+runtime creates a single one).

Updates #28486.

Change-Id: I01c571fee2376474c7f3038e801bd58fd9e0b820
Reviewed-on: https://go-review.googlesource.com/c/150097


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