[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>