cmd/compile: simplify walk OCONVIFACE

cmd/compile: simplify walk OCONVIFACE

n.Type and n.Left.Type are used heavily. Give them useful names.

We generate the type word frequently. Make it a closure.
(We don't want to generate it up front, since there are some code
paths that don't need it, and generating it has side-effects.)

Simplify and document the final call construction.

Follow-up to address feedback on CL 147360.

Change-Id: I251134a55cf80d8b1676280a345d150f2288c09a
Reviewed-on: https://go-review.googlesource.com/c/147538
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Martin Möhrmann <moehrmann@google.com>