cmd/compile/internal/types2: remove most asX converters (cleanup)

Make it explicit in the code where we call under.
The asNamed and asTypeParam converters need to
stay: asNamed does resolution if necessary, and
asTypeParam may or may not call under() depending
on the next CL.

Reviewed uses of asNamed and .(*Named) for correctness.
Removed unnecessary Named.resolve call in lookup.

Change-Id: I2acf176925e00bd1703a00230a779aa65a8f5a51
Reviewed-on: https://go-review.googlesource.com/c/go/+/362254


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>