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

This CL renames the toT converters back to their asT names.

Change-Id: If4bda5a78525f9722f044f5544f400fa8bb6f437
Reviewed-on: https://go-review.googlesource.com/c/go/+/358774


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