cmd/compile/internal/types2: implement compiler helper functions without using under

These functions are exported for the compiler and are used after
type checking is finished. There is no need to call under() in
their implementations; they can rely entirely on the public API.
This opens the door to moving them into the compiler eventually.
They may also be slightly more efficient.

Change-Id: Ib4f83d2dcf82e3c319c3147e01ecaea684553ea5
Reviewed-on: https://go-review.googlesource.com/c/go/+/361214


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