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

This reverts commit 759eaa22.

Reason to revert: break unified IR builder

Though the unified IR is not for go1.18, it's the only user of types2
lazy resolution APIs at this moment. And it consistently failed after
CL 362254 is the sign that the change was wrong somehow.

Change-Id: I6bfc3192904fe2129fd3c165f0df8761e8eb441c
Reviewed-on: https://go-review.googlesource.com/c/go/+/361964


Trust: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>