go/types: remove most asX converters (cleanup)

go/types: remove most asX converters (cleanup)

This is a port of CL 362118 to go/types, which is itself a roll-forward
of CL 362254, containing a bugfix.

Change-Id: I20067c7adf56bf64fe9ad080d998a7aefbdc1053
Reviewed-on: https://go-review.googlesource.com/c/go/+/362617
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>