This change fixes shadow assignment check in cases when RHS is not an identifier
or a type assertion.

Fixes #12188

Change-Id: I0940df8d9c237ab8b8d3272eb6895e676c75c115
Reviewed-on: https://go-review.googlesource.com/16038Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>