cmd/compile: more use of IterXXX functions

cmd/compile: more use of IterXXX functions

This CL was mostly produced by a one-off automated rewrite tool
looking for statements like "for X := T.Type; X != nil; X = X.Down"
and a few minor variations.

Passes toolstash -cmp.

Change-Id: Ib22705e37d078ef97841ee2e08f60bdbcabb94ad
Reviewed-on: https://go-review.googlesource.com/20520
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>