compiler: use correct assignment order for type assertions

For "a, b := v.(T)" we must set a before b.

For golang/go#13433

Change-Id: I68512ad933667b809ca935b204a18e4ad6a92d36
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/273906
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>