go/types, types2: remove unnecessary tests for x.typ == Typ[Invalid]
In the worst case (x.mode != invalid but x.typ == Typ[Invalid]) we
may get unexpected additional errors; but we don't seem to have
any such situations, at least in the existing tests.
Change-Id: I86ae607b4ac9b926264bb6a967627c40e5a86ade
Reviewed-on: https://go-review.googlesource.com/c/go/+/478715 Reviewed-by: Robert Findley <rfindley@google.com> Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>
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