cmd/compile: remove last manual node copies

cmd/compile: remove last manual node copies

When I added the Node.copy method, I converted most of the occurrences
but missed a few.

One of them, used only for gdata, was an unnecessary copy given that
gdata does not modify the node it is passed.

No allocation changes in compilebench.

Passes toolstash -cmp on std cmd.

Change-Id: I7fba5212377b75c6d6b3785e594a30568ff0732e
Reviewed-on: https://go-review.googlesource.com/104937
Run-TryBot: Daniel Martí <mvdan@mvdan.cc>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>