go/types: avoid declared but not used error for a couple of cases

go/types: avoid declared but not used error for a couple of cases

The change in typexpr.go matches types2 behavior.

For #54511.

Change-Id: I79c922a94f2ee0440c1814140935c321439c7d25
Reviewed-on: https://go-review.googlesource.com/c/go/+/425714
Reviewed-by: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>