[dev.regabi] cmd/compile: separate typecheck more cleanly

Abstract the typecheck API a bit more so that it is
easier to move into a new package.

Change-Id: Ia0a0146151fa7f6073113e68a2c3f6e42a5d0ad8
Reviewed-on: https://go-review.googlesource.com/c/go/+/279303


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>