go/types, types2: avoid recursive invocation when unifying underlying types

go/types, types2: avoid recursive invocation when unifying underlying types

There's no need to invoke unifier.nify recursively when we decide to
unify underlying types. Just update the respective type variable and
continue.

Change-Id: I3abe335464786dc509d18651dff14b20022c7d63
Reviewed-on: https://go-review.googlesource.com/c/go/+/464347
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Griesemer <gri@google.com>