cmd/compile/internal/types2: roll-forward removal of asX converters

cmd/compile/internal/types2: roll-forward removal of asX converters

This CL reverts CL 361964, rolling forward the original CL 362254 with a
fix for re-entrant expansion via type hashing (compare patchsets 1 and
2).

Change-Id: I62869e50e919f42eb8d6fef5b0d7a5ec8960bd84
Reviewed-on: https://go-review.googlesource.com/c/go/+/362118
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>