cmd/compile: cleanup walking OCONV/OCONVNOP

Use a separate func, which needs less indentation and can use returns
instead of labelled breaks. We can also give the types better names, and
we don't have to repeat the calls to conv and mkcall.

Passes toolstash -cmp on std cmd.

Change-Id: I1071c170fa729562d70093a09b7dea003c5fe26e
Reviewed-on: https://go-review.googlesource.com/130075


Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>