cmd/compile: refactor expandiface

Move checkdupfields call into expandiface, and inline/simplify offmod.
More prep work for implementing #6977.

Change-Id: I958ae87f761ec25a8fa7298a2a3019eeca5b25ba
Reviewed-on: https://go-review.googlesource.com/c/go/+/187518


Reviewed-by: Robert Griesemer <gri@golang.org>