cmd/compile: eliminate uses of Type.Down in alg.go

cmd/compile: eliminate uses of Type.Down in alg.go

This could be done by threading the Iter value down through memrun and
ispaddedfield, but that ends up a bit clunky. This way is also closer
to how we'll want the code to look once fields are kept in slices.

Passes toolstash -cmp.

Change-Id: I8a44445c85f921eb18d97199df2026c5ce0f4f67
Reviewed-on: https://go-review.googlesource.com/20558
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>