cmd/compile/internal/types2: clean up asT converters (step 1 of 2)

This CL changes the convenience converters asT to use under instead
of optype. To make sure the effect is well understood, in a first
step, all asT functions are renamed to toT so that we can see which
call sites are affected. In almost all places, the change is what we
want. In some places we may get more conservative behavior (which is
easy to relax if need be). In some places (function calls through a
type parameter, append built-in) we now use singleUnder instead, for
a more general behavior, matching other primary expressions or built-
ins.

This change removes the last use of optype and thus also theTop and
top, all of which have been deleted from the code.

The next CL renames the toT converters back to their asT form.

Change-Id: I35d1ad866ce46de175a055b36ef577d99bb9de22
Reviewed-on: https://go-review.googlesource.com/c/go/+/358597


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