The unification code has "early exits" when the compared
types are pointer-identical.

Because of Alias nodes, we cannot simply compare x == y but we
must compare Unalias(x) == Unalias(y). Still, in the common case
there are no aliases, so as a minor optimization we write:

        x == y || Unalias(x) == Unalias(y)

to test whether x and y are (pointer-) identical.
Add the missing Unalias calls in the place where we forgot them.

Fixes #67872.

Change-Id: Ia26ffe7205b0417fc698287a4aeb1c900d30cc0d
Reviewed-on: https://go-review.googlesource.com/c/go/+/591975


Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Griesemer <gri@google.com>