cmd/compile/internal/types2: simplify Named.under

Remove some unnecessary logic from Named.under:
 - no need to have special handling for Typ[Invalid]: this is the same
   as other cases where the underlying type is resolved.
 - use Underlying() to get the loaded and expanded underlying
 - no need for special handling of the first iteration

Change-Id: I2029711f51fa9eaaee11debadd55974a1376a980
Reviewed-on: https://go-review.googlesource.com/c/go/+/341857


Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>