cmd/compile/internal/types2: rename isNamed predicate to hasName

cmd/compile/internal/types2: rename isNamed predicate to hasName

isNamed(t) is easily confused with asNamed(t) != nil (e.g., we
have isPointer(t) that is defined as asPointer(t) != nil).

This rename also helped clarifying a couple of places in the
assignability rules where it makes sense to simply look for
types that have names.

Change-Id: Ie995908613a26883ffe0562343d297a1e981e9ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/358621
Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>