cmd/compile: eliminate a bunch of IterFields/IterMethods calls

cmd/compile: eliminate a bunch of IterFields/IterMethods calls

This is an automated rewrite of all the calls of the form:

    for f, it := IterFields(t); f != nil; f = it.Next() { ... }

Followup CLs will work on cleaning up the remaining cases.

Change-Id: Ic1005ad45ae0b50c63e815e34e507e2d2644ba1a
Reviewed-on: https://go-review.googlesource.com/20794
Reviewed-by: David Crawshaw <crawshaw@golang.org>