[dev.typeparams] cmd/compile: fix case where we were copying a raw Node

Replace the raw Node copy with the creation of a new node, and the
copying of the needed flags and fields.

Change-Id: I636bf228ba28c0d5dc25f8366d82379d86ecd731
Reviewed-on: https://go-review.googlesource.com/c/go/+/322189


Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Dan Scales <danscales@google.com>