go/types: use same local variable consistently (minor cleanup)

Currently this CL has no effect because V == x.typ in the affected
code. But if we should ever manipulate V (e.g., to support some form
of lazy evaluation of the type), not using V consistently would
lead to a subtle bug.

Change-Id: I465e72d18bbd2b6cd8fcbd746e0d28d14f758c03
Reviewed-on: https://go-review.googlesource.com/c/go/+/228105


Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>