cmd/compile/internal/gc: fix Node.copy and introduce (raw|sep)copy

Node.copy used to make a shallow copy of a node. Often, this is not
correct: If a node n's Orig field pointed to itself, the copy's Orig
field has to be adjusted to point to the copy. Otherwise, if n is
modified later, the copy's Orig appears modified as well (because it
points to n).

This was fixed for one specific case with
https://go-review.googlesource.com/c/go/+/136395 (issue #26855).

This change instead addresses copy in general:

In two cases we don't want the Orig adjustment as it causes escape
analysis output to fail (not match the existing error messages).
rawcopy is used in those cases.

In several cases Orig is set to the copy immediately after making
a copy; a new function sepcopy is used there.

Updates #26855.
Fixes #27765.

Change-Id: Idaadeb5c4b9a027daabd46a2361348f7a93f2b00
Reviewed-on: https://go-review.googlesource.com/136540


Reviewed-by: Matthew Dempsky <mdempsky@google.com>