cmd/compile: avoid leak of dottype expression on double assignment form

cmd/compile: avoid leak of dottype expression on double assignment form

This is a followup to issue #13805. That change avoid leaks for types that
don't have any pointers for the single assignment form of a dottype expression.
This does the same for the double assignment form.

Fixes #15796

Change-Id: I27474cade0ff1f3025cb6392f47b87b33542bc0f
Reviewed-on: https://go-review.googlesource.com/24906
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>